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^D22. 



EDWARD NELSON DINGLEY 
COLLECTION 

PRESENTED BY HIS WIFE 



THE 

POWER OF MOVEMENT 
IN PLANTS 



BY 

HARLES DARWIN, LL. D., F.R.S. 



ASSISTED BY 

FRANCIS DARWIN 



WITH ILLUSTRATIONS 



NEW YORK 

D. APPLETON AND COMPANY 

1896 



QK77I 

I23G 



Authorized Edition. 

C f .rcrr 
Mr§. Edwarb N. Dingle* 
July 11 1932 



CONTENTS 



Lnteoduction Page 1-9 

CHAPTER I. 

The Circumnutating Movements of Seedling Plants. 

Brassica oleracea, circumnutation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above 
the ground and straightening itself, and when erect — Circumnu- 
tation of the cotyledons — Kate of movement — Analogous obser- 
vations on various organs in species of Githago, Gossypium, 
Oxalis, Tropseolum, Citrus, iEsculus, of several Leguminous and 
Cucurbitaceous genera, Opuntia, Helianthus, Primura, Cyclamen, 
Stapelia, Cerinthe, Nolana, Solanum, Beta, Eicinus, Quercus, 
Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, 
A vena, Nephrodium, and Selaginella 10-66 

CHAPTER II. 

General Considerations on the Movements and Growth of 
Seedling Plants. 

Generality of the circumnutating movement — Eadicles, their cir- 
cumnutation of service — Manner in which they penetrate the 
ground — Manner in which hypocotyls and other organs bieak 
through the ground by being arched — Singular manner of ger- 
mination in Megarrhiza, &c— Abortion of cotyledons — Circum- 
nutation of hypocotyls and epicotyls whilst still buried and 
arched — Their power of straightening themselves— Bursting of 
the seed-ccats — Inherited effect of the arching process in hypo- 



VI CONTENTS. 

gean hypocotyls — Circumnutation of hypocotyls and epicotyl? 
when erect — Circumnutation of cotyledons — Pulvini or joints of 
cotyledons, duration of their activity, rudimentary in Oxalis 
corniculata, their development — Sensitiveness of cotyledons to 
light and consequent disturbance of their periodic movements — 
Sensitiveness of cotyledons to contact Page 67-128 



CHAPTER III. 

Sensitiveness of the Apex of the Radicle to Contact and 
to other Irritants. 

Maimer in which radicles bend when they encounter an obstacle in 
the soil — Yicia faba, tips of radicles highly sensitive to con- 
tact and other irritants — Effects of too high a temperature — 
Power of discriminating between objects attached on opposite 
sides — Tips of secondary radicles sensitive — Fisum, tips of 
radicles sensitive — Effects of such sensitiveness in overcoming 
geotropism — Secondary radicles — Phaseolus, tips of radicles 
hardly sensitive to contact, but highly sensitive to caustic and 
to the removal of a slice — Tropseolurn — Gossypium-^-Cucurbita 
— Raphanus— ^E^culus, tip not sensitive to slight contact, highly 
sensitive to caustic — Quercus, tip highly sensitive to contact — 
Power of discrimination — Zea, tip highly sensitive, secondary 
radicles — Sensitiveness of radicles to moist air — Summary of 
chapter 129-200 



CHAPTER IY. 

The Circoinutating Movements of the several parts of 
Mature Plants. 

Circumnutation of stems: concluding remarks on — Circumnutation 
of stolons: aid thus afforded in winding amongst the stems of 
surrounding plants — Circumnutation of flower-stems — Circum- 
nutation of Dicotyledonous leaves — Singular oscillatory move- 
ment }f leaves of Dionsea — Leaves of Cannabis sink at night — 
Leaves of Gymnosperms — Of Monocotyledons — Cryptogams — 
Concluding remarks on the circumnutation of leaves : generally 
rise in the evening and sink in the morning .. .. 201-262 



CONTENTS. V11 

CHAPTER V. 

modified circumnutation .* climbing plants; epinastic and 
Hyponastic Movements. 

Circumautation modified through innate causes or through the action 
of external conditions — Innate causes — Climbing plants ; simi- 
larity of their movements with those of ordinary plants; in- 
creased amplitude; occasional points of difference — Epinastic 
growth of young leaves — Hyponastic growth of the hypocotyls 
and epicotyls of seedlings — Hooked tips of climbing and other 
plants due to modified circumnutation — Ampelopsis tricuspidata 
— Smithia Pi'undii — Straightening of the tip due to hyponasty — 
Epinastic growth and circumnutation of the flower-peduncles of 
Trifolium repens and Oxalis carnosa Page 263-279 

CHAPTER VI. 

Modified Circumnutation: Sleep or Nyctitropic Movements, 
their Use: Sleep of Cotyledons. 

Preliminary sketch of the sleep or nyctitropic movements of leaves 
— Presence of pulvini — The lessening of radiation the final cause 
of nictri tropic movements — Manner of trying experiments on 
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, 
and on the cotyledons of Mimosa — Concluding remarks on radia- 
tion from leaves — Small differences in the conditions make a 
great difference in the result — Description of the nyctitropic 
position and movements of the cot}ledons of various plants — 
List of species — Concluding remarks — Independence of the 
nyctitropic movements of the leaves and cotyledons of the same 
species — Reasons for believing that the movements have been 
acquired for a special purpose 280-316 

CHAPTER VII. 

Modified Circumnutation : Nyctitropic or Sleep Movements 
of Leaves. 

Conditions necessary for these movements — List of Genera and 
Families, which include sleeping plants — Description of tho 
movements in the several Genera — Oxalis : leaflets folded at 



Vill CONTENTS. 

night — Averrhoa: rapid movements of the leaflets — Porlieria'. 
leaflets close when plant kept very dry — Tropa?olum : leaves do 
not sleep unless well illuminated during day — Lupinus : various 
modes of sleeping — Melilotus : singular movements of terminal 
leaflet — Trifolium — Desmodium : rudimentary lateral leaflets, 
movements of, not developed on young plants, state of their 
pulvini — Cassia : complex movements of the leaflets — Bauhinia: 
leaves folded at nuht — Mimosa pudica : compounded move- 
ments of leaves, effect of darkness — Mimosa albida, reduced 
leaflets of — Schrankia : downward movement of the pinna? — 
Marsilea : the only cryptogam known to sleep — Concluding 
remarks and summary — Nyctitropism consists of modified cir- 
cumnutation, regulated by the alternations of light and darkness 
— Shape of fiist true leaves Page 317-417 



CHAPTEE VIII. 

Modified Circumnutation : Movements excited by Light. 

Distinction between heliotropism and the effects of light on the 
periodicity of the movements of leaves — Heliotropic movements 
of Beta, Solanum, Zea, and Avena — Heliotropic movements 
towards an obscure light in Apios, Brassica, Phalaris, Tropseo- 
lum, and Cassia — Apheliotropic movements of tendrils of Big- 
nonia — Of flower-peduncles of Cyclamen — Burying of the pods 
— Heliotropism and apheliotropism modified forms of circumnu- 
tation — Steps by which one movement is converted into the 
other — Transversal-heliotropismus or diaheliotropism influenced 
by epinasty, the weight of the part and apogeotropism — Apogeo- 
tropism overcome during the middle of the day by diaheliotro- 
pism — Effects of the weight of the blades of cotyledons — So- 
called diurnal sleep — Chlorophyll injured by intense light — 
Movements to avoid intense light 418-448 

CHAPTER IX. 

Sensitiveness of Plants to Light : its transmitted effects. 

Uses of he'iotropism — Insectivorous and climbing plants not helio- 
tropic — Same organ heliotropic at one age and not at another — 
Extraordinary sensitiveness of some plants to light — The effects 



CONTENTS. IS 

of light do not correspond with its intensity — E fleets of previous 
illumination — Time required for the action of light — After-effects 
of light — Apogeotropism acts as soon as light fails — Accuracy 
with which plants benfl. to the light — This dependent on the 
illumination of one whole side of the part — Localised sensitive- 
ness to light and its transmitted effects — Cotyledons of Phalaris, 
manner of bending — Results of the exclusion of light from their 
tips — Effects transmitted beneath the surface of the ground — 
Lateral illumination of the tip determines the direction of the 
curvature of the base — Cotyledons of Avena, curvature of basal 
part due to the illumination of upper part — Similar results with 
the hypocotyls of Brassica and Beta — Radicles of Sinapis aphelio- 
tropic, due to the sensitiveness of their tips — Concluding remarks 
and summary of chapter — Means by which circumnutation han 
been converted into heliotropism or apheliotropism Page 449-492 



CHAPTER X. 

Modified Circumnutation : Movements excited by 
Gravitation. 

Means of observation — Apogeotropism — Cytisus — Verbena — Beta 
— Gradual conversion of the movement of circumnutation into 
apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica 
— Apogeotropism retarded by heliotropism — Effected by the aid 
of joints or pulvini — Movements of flower- peduncles of Oxalis — 
General remarks on apogeotropism — Geotropism — Movements of 
radicles — Burying of seed-capsules — Use of process — Trifolium 
subterraneum — Arachis — Amphicarpasa — Diageotropism — 
Conclusion 493-522 



CHAPTER XL 

Localised Sensitiveness to Gravitation, and its Transmitter 
Effects. 

General considerations — Vicia faba, effects of amputating the tips of 
the radicles — Regeneration of the tips — Effects of a short ex- 
posure of the tips to geotropic action and their subsequent 
amputation — Kffects of amputating the tips obliquely — Effects 
of cauterising the tips — Effects of grease on the tips — Pisum 



CONTENTS. 

sativum, tips of radicles cauterised transversely, and on theii 
upper and lower sides— Phaseolus, cauterisation and grease on 
the tips — Gossypium — Cucurbita, tips cauterised transversely, 
and on their upper and lower sides — Zea, tips cauterised — Con- 
cluding remarks and summary of chapter — Advantages of the 
sensibility to geotropism being localised in the tips of the 
radicles Page 23-545 



CHAPTER XII. 

Summary and Concluding Remarks. 

Nature of the circum nutating movement — History of a germinating 
seed — The radicle first protrudes and circumnutates — Its tip 
highly sensitive — Emergence of the hypocotyl or of the epicotyl 
from the grcund under the form of an arch — Its circumnutation 
and that of the cotyledons — The seedling throws up a leaf- 
bearing stem — The circumnutation of all the parts or organs — 
Modified circumnutation — Epinasty and hyponasty — Movements 
of climbing plants — Nyctitropic movements — Movements excited 
by light and gravitation — Localised sensitiveness — Resemblance 
between the movements of plants and animals — The tip of the 
radicle acts like a brain 546-573 

Index 574-593 



THE MOVEMENTS OF PLANTS. 



INTRODUCTION. 

The chief object o£ the present work is to describe 
and connect together several large classes of move- 
ment, common to almost all plants. The most widely 
prevalent movement is essentially of the same nature 
as that of the stem of a climbing plant, which bends 
successively to all points of the compass, so that the 
tip revolves. This movement has been called by 
Sachs " revolving nutation ;" but we have found it 
much more convenient to use the terms circumnutation 
and circumnutate. As we shall have to say much 
about this movement, it will be useful here briefly to 
describe its nature. If we observe a circumnutating 
stem, which happens at the time to be bent, we will 
say towards the north, it will be found gradually to 
bend more and more easterly, until it faces the east ; 
and so onwards to the south, then to the west, and 
back again to the north. If the movement had been 
quite regular, the apex would have described a circle, 
or rather, as the stem is always growing upwards, a 
circular spiral. But it generally describes irregular 
elliptical or oval figures ; for the apex, after point- 
ing in any one direction, commonly moves back 
to the opposite side, not, however, returning along 
the same line. Afterwards other irregular ellipses 
or ovals are successively described, with their longer 



2 INTRODUCTION. 

axes directed to different points of the compass. 
Whilst describing such figures, the apex often travels 
in a zigzag line, or makes small subordinate loops or 
triangles. In the case of leaves the ellipses are 
generally narrow. 

Until recently the cause of all such bending move- 
ments was believed to be due to the increased growth 
of the side which becomes for a time convex ; that this 
side does temporarily grow more quickly than the 
concave side has been well established ; but De Vries 
has lately shown that such increased growth follows 
a previously increased state of turgescence on the 
convex side.* In the case of parts provided with a 
so-called joint, cushion or pulvinus, which consists of 
an aggregate of small cells that have ceased to 
increase in size from a very early age, we meet with 
similar movements; and here, as Pfeffer has shown f 
and as we shall see in the course of this work, 
the increased turgescence of the cells on opposite 
sides is not followed by increased growth. Wiesner 
denies in certain cases the accuracy of De Vries' con- 
clusion about turgescence, and maintains :f that the 
increased extensibility of the cell- walls is the more 
important element. That such extensibility must 
accompany increased turgescence in order that the part 
may bend is manifest, and this has been insisted on by 
several botanists ; but in the case of unicellular plants 
it can hardly fail to be the more important element. 
Oi the whole we may at present conclude that in- 



* Sachs first showed "Lehr- 19, 1879, p. 830. 

buch,' &c, 4th edit. p. 452) the f 'D;e Periodiscben Bewegun- 

intimate connection between tur- gen der Blattorgane,' 1875. 

g< scence and growth. For De I 'Untersuchungen iiber den 

Vries* interesting essay, ' Wachs- Heliotropismus,' Sitzb. der K, 

thumskrurnmungen mehrzelliger Akad. derWissenschaft. (Vienna). 

Organe,' see • Bot. Zeitung,' Dec. Jan. 1880. 



INTRODUCTION. ' O* 

creased growth, first ov one side and then on another, 
is a secondary effect, and that the increased tnr- 
gescence of the cells, together with the extensibility 
of their walls, is the primary cause of the movement of 
circumnutation.* 

In the course of the present volume it will be shown 
that apparently every growing part of every plant is 
continually circumnutating, though often on a small 
scale. Even the stems of seedlings before they have 
broken through the ground, as well as their buried 
radicles, circum nutate, as far as the pressure of the 
surrounding earth permits. In this universally pre- 
sent movement we have the basis or groundwork for 
the acquirement, according to the requirements of the 
plant, of the most diversified movements. Thus, the 
great sweeps made by the stems of twining plants, 
and by the tendrils of other climbers, result from 
a mere increase in the amplitude of the ordinary 
movement of circumnutation. The position which 
young leaves and other organs ultimately assume 
is acquired by the circumnutating movement being 
increased in some one direction. The leaves of 
various plants are said to sleep at night, and it will 
be seen that their blades then assume a vertical 
position through modified circumnutation, in order 
to protect their upper surfaces from being chilled 
through radiation. The movements of various organs 
to the light, which are so general throughout the 
vegetable kingdom, and occasionally from the light, 
or transversely with respect to it, are all modified 



* See Mr. Vines excellent dis- Naturkimde in Wurtemberg,' 
cussion (' Arbeiton des Bot. Insti- 1874, p. 211) on the curious move- 
tuts in Wi'uzburg,' B. II. pp 142, ments of Spirogyra, a plant con- 
143, 1878) on this intricate subject. sisting of a single row of cells,.aro 
Hofrneister's observations ('J;ih- valuable in relation to this subject, 
reschrifte des Vereins fur Yaterl. 



4 INTRODUCTION. 

forms of circumnutation ; as again are the equally 
prevalent movements of stems, &c, towards the zenith, 
and of roots towards the centre of the earth. In 
accordance with these conclusions, a considerable diffi- 
culty in the way of evolution is in part removed, for 
it might have been asked, how did all their diversified 
movements for the most different purposes first arise ? 
As the case stands, we know that, there is always 
movement in progress, and its amplitude, or direc- 
tion, or both, have only to be modified for the good 
of the plant in relation with internal or external 
stimuli. 

Besides describing the several modified forms of 
circumnutation, some other subjects will be discussed. 
The two which have interested us most are, firstly, the 
fact that with some seedling plants the uppermost 
part alone is sensitive to light, and transmits an influ- 
ence to the lower part, causing it to bend. If there- 
fore the upper part be wholly protected from light, 
the lower part may be exposed for hours to it, and yet 
does not become in the least bent, although this would 
have occurred quickly if the upper part had been 
excited by light. Secondly, with the radicles of seed- 
lings, the tip is sensitive to various stimuli, espe- 
cially to very slight pressure, and, when thus excited, 
transmits an influence to the upper part, causing it to 
bend from the pressed side. On the other hand, if 
the tip is subjected to the vapour of water proceeding 
from one side, the upper part of the radicle bends 
towards this side. Again it is the tip, as stated by 
Ciesielski, though denied by others, which is sensitive 
to the attraction of gravity, and by transmission causes 
the adjoining parts of the radicle to bend towards the 
centre of the earth. These several cases of the effects 
of contact, other irritants, vapour, light, and the 



INTRODUCTION. 5 

attraction of gravity being transmitted from the ex- 
cited part for some little distance along the organ in 
question, have an important bearing on the theory of 
all such movements. 

Terminology. — A brief explanation of some terms which will 
he used, must here be given. With seedlings, the stem which 
supports the cotyledons (i.e. the organs which represent the first 
leaves) has been called by many botanists the hypocotyledonous 
stem, but for brevity sake we will speak of it merely as the 
hypocotyl: the stem immediately above the cotyledons will be 
called the epicc-fyl or plumule. The radich can be distinguished 
from the hypocotyl only by the presence of root-hairs and the 
nature of its covering. The meaning of the word circumnu- 
tation has already been explained. Authors speak of positive 
and negative licliotropism,* — that is, the bending of an organ 
to or from the light ; but it is much more convenient to confine 
the word htliotropism to bending towards the light, and to 
designate as apheliotropism bending from the light. There is 
another reason for this change, for writers, as we have 
observed, occasionally drop the adjectives positive and negative, 
and thus introduce confusion into their discussions. Diahelio- 
tropism may express a position more or less transverse to 
the light and induced by it. In like manner positive geotro- 
pism, or bending towards the centre of the earth, will be 
called by us geotropism ; upoyeotropi^m will mean bending in 
opposition to gravity or from the centre of the earth; and dia- 
jeotropism, a position more or less transverse to the radius of 
the earth. The words heliotropism and geotropism properly 
mean the act of moving in relation to the light or the earth ; 
but in the same manner as gravitation, though defined as " the 
act of tending to the centre," is often used to express the cause 
of a body falling, so it will be found convenient occasionally to 
employ heliotropism and geotropism, &c, as the cause of the 
movements in question. 

The term epinasty is now often used in Germany, and implies 
that the upper surface of an organ grows more quickly than the 



* The highly useful terms of Frank : see his remarkable ' Bei- 
Heliotrop'sm and Gtotropism ti'age zur Pflauzenphysiologie, 
were first used bv Dr. A. B. 1868. 



6 INTRODUCTION. 

lower surface, and thus causes it to bend downwards. Hypo- 
nasty is the reverse, and implies increased growth along the 
lower surface, causing the part to bend upwards.* 

M>thods of Observation. — The movements, sometimes very 
small and sometimes considerable in extent, of the various 
organs observed by us, were traced in the manner which after 
many trials we found to be best, and which must be described. 
Plants growing in pots were protected wholly from the light, 
or had light admitted from above, or on one side as the case 
might require, and were covered above by a large horizontal 
sheet of glass, and with another vertical sheet on one side. A 
glass filament, not thicker than a horsehair, and from a quarter 
to three-quarters of an inch in length, was affixed to the part to 
be observed by means of shellac dissolved in alcohol. The 
solution was allowed to evaporate, until it became so thick that 
it set hard in two or three seconds, and it never injured the 
tissues, even the tips of tender radicles, to which it was applied. 
To the end of the glass filament an excessively minute bead of 
black sealing-wax was cemented, below or behind which a bit of 
card with a black dot was fixed to a stick driven into the ground. 
The weight of the filament was so slight that even small leaves 
were not perceptibly pressed down. Another method of obser- 
vation, when much magnification of the movement was not 
required, will presently be described. The bead and the dot 
on the card were viewed through the horizontal or vertical 
glass-plate (according to the position of the object), and when 
one exactly covered the other, a dot was made on the glass-plate 
with a sharply pointed stick dipped in thick Indian-ink. Other 
dots were made at short intervals of time and these were after- 
wards joined by straight lines. The figures thus traced were 
therefore angular; but if dots had been made every 1 or 
2 minutes, the lines would have been more curvilinear, as 
occurred when radicles were allowed to trace their own 
courses on smoked glass-plates. To make the dots accurately 
was the sole difficulty, and required some practice. Nor could 
this be done quite accurately, when the movement was much 
magnified, such as 30 times and upwards; yet even in this 
case the general course may be trusted. To test the accuracy 
of the above method of observation, a filament was fixed to an 



* These terms are used in the ' Wiirzburg Aibeiten,' Heft ii. 
•ease given them by De Vries, 1872, p. 252. 



INTRODUCTION. 7 

inanimate object which was made to siide along a straight 
edge and dots were repeatedly made on a glass-plate; when 
these were joined, the result ought to have been a perfectly 
straight line, and the line was very nearly straight. It may be 
added that when the dot on the card was placed half-an-inch 
below or behind the bead of sealing-wax, and when the glass- 
plate (supposing it to have been properly curved) stood at a 
distance of 7 inches in front (a common distance), then the 
tracing represented the movement of the bead magnified 15 
times. 

Whenever a great increase of the movement was not required, 
another, and in some respects better, method of observation was 
followed. This consisted in fixing two minute triangles of thin 
paper, about ■£$ inch in height, to the two ends of the attached 
glass filament ; and when their tips were brought into a line so 
that they covered one another, dots were made as before on the 
glass-plate. If we suppose the glass-plate to stand at a dis- 
tance of seven inches from the end of the shoot bearing the 
filament, the dots when joined, will give nearly the same figure 
as if a filament seven inches long, dipped in ink, had been 
fixed to the moving shoot, and had inscribed its own course 
on the plate. The movement is thus considerably magnified; 
for instance, if a shoot one inch in length were bending, and 
the glass-plate stood at the distance of seven inches, the move- 
ment would be magnified eight times. It would, however, have 
been very difficult to have ascertained in each case how great 
a length of the shoot was bending; and this is indispensable 
for ascertaining the degree to which the movement is magnified. 

After dots had been made on the glass-plates by either of 
the above methods, they were copied on tracing paper and 
joined by ruled lines, with arrows showing the direction of the 
movement. The nocturnal courses are represented by straight 
broken lines. The first dot is always made larger than the 
others, so as to catch the eye, as may be seen in the diagrams. 
The figures on the glass-plates were often drawn on too large 
a scale to be reproduced on the pages of this volume, and the 
proportion in which they have been reduced is always given.* 
Whenever it could be approximately told how much the move- 
ment had been magnified, this is stated. We have perhaps 



* We are much indebted to he has reduced and engraved out 
Mr. Cooper fur the care with which diagrams. 



8 INTRODUCTION. 

introduced a superfluous number of diagrams; but they take 
up less space than a full description of the movements. Almost 
all the sketches of plants asleep, &c, were carefully drawn 
for us by Mr. George Darwin. 

As shoots, leaves, &c, in circumnutating bend more and 
more, first in one direction and then in another, they were 
necessarily viewed at different times more or less obliquely; 
and as the dots were made on a flat surface, the apparent 
amount of movement is exaggerated according to the degree 
of obliquity of the point of view. It would, therefore, have 
been a much better plan to have used hemispherical glasses, 
if we had possessed them of all sizes, and if the bending part 
of the shoot had been distinctly hinged and could have been 
placed so as to have formed one of the radii of the sphere- 
But even in this case it would have been necessary afterwards 
to have projected the figures on paper; so that complete 
accuracy could not have been attained. From the distortion 
of our figures, owing to the above causes, they are of no use 
to any one who wishes to know the exact amount of movement, 
or the exact course pursued; but they serve excellently for 
ascertaining whether or not the part moved at all, as well as 
the general character of the movement. 



In the following chapters, the movements of a con- 
siderable number of plants are described ; and the 
species have been arranged according to the system 
adopted by Hooker in Le Maout and Decaisne's * De- 
scriptive Botany.' No one who is not investigating 
the present subject need read all the details, which, 
however, we have thought it advisable to give. To 
save the reader trouble, the conclusions and most of 
the more important parts have been printed in larger 
type than the other parts. He may, if he thinks fit, 
read the last chapter first, as it includes a summary 
of the whole volume ; and he will thus see what 
points interest him, and on which he requires the 
full evidence. 

Finally, we must have the pleasure of returning oiu 



INTRODUCTION. 9 

sincere thanks to Sir Joseph Hooker and to Mr. W. 
Thiselton Dyer for their great kindness, in not only 
sending us plants from Kew, but in procuring others 
from several sources when they were required for our 
observations ; also, for naming many species, and giving 
us in formation on various points. 



10 CIRCUMNUTATION OF SEEDLINGS. Chap, t 



CHAPTER I. 

The Circtjmntjtattng Movements of Seedling Plants. 

Brassica oleracea, circumnutation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above the 
ground and straightening itself, and when erect — Circumnutation 
of the cotyledons — Rate of movement — Analogous observations on 
various organs in species of Githago, Gossypium, Oxalis, Tro- 
pseolum, Citrus, JEsculus, of several Leguminous and Cucurbita- 
ceous genera, Opuntia, Helianthus, Primula, Cyclamen, StapeLa, 
Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus, 
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, A vena, Nephro- 
dium, and Selagiuella. 

The following chapter is devoted to the circum- 
nutating movements of the radicles, hypocotyls, and 
cotyledons of seedling plants ; and, when the coty- 
ledons do not rise above the ground, to the movements 
of the epicotyl. But in a future chapter we shall have 
to recur to the movements of certain cotyledons which 
sleep at night. 

Brassica oltracea (Cruciferce). — Puller details will be given 
with respect to the movements in this case than in any other, 
as space and time will thus ultimately be saved. 

Radicle. — A seed with the radicle projecting -05 inch was 
fastened with shellac to a little plate of zinc, so that the 
radicle stood up vertically; and a fine glass filament was then 
fixed near its base, that is, close to the seed-coats. The seed 
was surrounded by little bits of wet sponge, and the move- 
ment of the bead at the end of the filament was traced (Fig. 1) 
during sixty hours. In this time the radicle increased in 
length from '05 to "11 inch. Had the filament been attached at 
first close to the apex of the radicle, and if it could have re- 
mained there all the time, the movement exhibited would have 



Chap. I, 



BEASSIOA. 



11 



^> 



been much greater, for at the close of our observations the tip, 
instead of standing vertically upwards, had become bowed 
downwards through geotropism, so as almost to touch the zinc 
plate. As far as we could 
roughly ascertain by measure- Fi S« l • 

ments made with compasses 
on other seeds, the tip alone, 
for a length of only y§o to 
y§o of an inch, is acted on 
by geotropism. But the trac- 
ing shows that the basal part 
of the radicle continued to 
circumnutate irregularly dur- 
ing the whole time. The 
actual extreme amount of 
movement of the bead at the 
end of the filament was nearly 
•05 inch, but to what extent 
the movement of the radicle 
was magnified by the fila- 
ment, which was nearly f inch 
in length, it was impossible 
to estimate. 

Another seed was treated and observed in the same manner, 
but the radicle in this case protruded -1 inch, and was not 




Brassca oleriacea : circumnutation of 
radicle, traced on horizontal glass, 
from 9 A.M. Jan. 31st to 9 P.M. 
Feb. 2nd. Movement of bead at 
end of filament magnified about 
40 times. 



Fig. 2. 




\ 
Brassica okracea : circumnutating and geotropic movement of radicle, 
traced on horizontal glass during 46 hours. 

fastened so as to project quite vertically upwards. The filament 
was affixed close to its base. The tracing (Fig. 2, reduced by 
half) shows the movement from 9 a.m. Jan. 31st to 7 a.m. 
Feb. 2nd; but it continued to move during the whole of the 



12 CTRCUMNUTATION OF SEEDLINGS. Chap. 1. 

2nd in the same general direction, and in a similar zigzag 
manner. From the radicle not being quite perpendicular when 
the filament was affixed geotropism came into play at once; 
but the irregular zigzag course shows that there was growth 
(probably preceded by turgescence), sometimes on one and 
sometimes on another side. Occasionally the bead remained 
stationary for about an hour, and then probably growth occurred 
on the side opposite to that which caused the geotropic curva- 
ture. In the case previously described the basal part of the 
very short radicle from being turned vertically upwards, was at 
first very little affected by geotropism. Filaments were affixed 
in two other instances to rather longer radicles protruding 
obliquely from seeds which had been turned upside down ; and 
in these cases the lines traced on the horizontal glasses were 
only slightly zigzag, and the movement was always in the same 
general direction, through the action of geotropism. All these 
observations are liable to several causes of error, but we believe, 
from what will hereafter be shown with respect to the move- 
ments of the radicles of other plants, that they may be largely 
trusted. 

Hyipocotyl. — The hypocotyl protrudes through the seed-coats 
as a rectangular projection, which grows rapidly into an arch 
like the letter U turned upside down [\ ; the cotyledons being 
still enclosed within the seed. In whatever position the seed 
may be embedded in the earth or otherwise fixed, both legs of 
the arch bend upwards through apogeotropism, and thus rise 
vertically above the ground. As soon as this has taken place, 
or even earlier, the inner or concave surface of the arch grows 
more quickly than the upper or convex surface; and this tends 
to separate the two legs and aids in drawing the cotyledons out 
of the buried seed-coats. By the growth of the whole arch the 
cotyledons are ultimately dragged from beneath the ground, even 
from a considerable depth; and now the hypocotyl quickly 
straightens itself by the increased growth of the concave side. 

Even whilst the arched or doubled hypocotyl is still beneath 
the ground, it circumnutates as much as the pressure of the sur- 
rounding soil will permit; but this was difficult to observe, 
because as soon as the arch is freed from lateral pressure the two 
legs begin to separate, even at a very early age, before the arch 
would naturally have reached the surface. Seeds were allowed 
to germinate on the surface of damp earth, and after they had 
fixed themselves by their radicles, and after the, as yet, only 



Chap. I. BKASS1CA. 13 

slightly arched hypocotyl had become nearly vertical, a glass 
filament was affixed on two occasions near to the base of the 
basal leg (i.e. the one in connection with the radicle), and its 
movements were traced in darkness on a horizontal glass. The 
result was that long lines were formed running in nearly the 
plane of the vertical arch, due to the early separation of the 
two legs now freed from pressure ; but as the lines were zigzag, 
showing lateral movement, the arch must have been circum- 
nutating, whilst it was straightening itself by growth along its 
inner or concave surface. 
A somewhat different method of observation was next followed : 

Fig. 3. 



Brassica oleracea : circumnutating movement of buried and arched hypo- 
cotyl (dimly illuminated from above), traced on horizontal glass during 
45 hours. Movement of bead of filament magnified about 25 times, 
' and here reduced to one-half of original scale. 

as soon as the earth with seeds in a pot began to crack, the 
surface was removed in parts to the depth of 2 inch ; and a 
filament was fixed to the basal leg of a buried and arched hypo- 
cotyl, just above the summit of the radicle. The cotyledons 
were still almost completely enclosed within the much-cracked 
seed-coats ; and these were again covered up with damp adhesive 
soil pressed pretty firmly down. The movement of the filament 
was traced (Fig. 3) from 11 a.m. Feb. 5th till 8 a.m. Feb. 7th. 
By this latter period the cotyledons had been dragged from 
beneath the pressed-down earth, but the upper part of the 
hypocotyl still formed nearly a right angle with the lower part. 
The tracing shows that the arched hypocotyl tends at this early 



14 CIKCUMNUTATION OF SEEDLINGS. Chap. I 

age to circumnutate irregularly. On the first day the greatei 
movement (from right to left in the figure) was not in the plane 
of the vertical and arched hypocotyl, but at right angles to it, or in 
the plane of the two cotyledons, which were still in close contact. 
The basal leg of the arch at the time when the filament was 
affixed to it, was already bowed considerably backwards, or 
from the cotyledons ; had the filament been affixed before this 
bowing occurred, the chief movement would have been at right 
angles to that shown in the figure. A filament was attached to 
another buried hypocotyl of the same age, and it moved in a 
similar general manner, but the line traced was not so complex. 
This hypocotyl became almost straight, and the cotyledons were 
dragged from beneath the ground on the evening of the second day 

Fig. 4. 




Brassica oleracea : circumnutating movement of buried and arched hypo- 
cotyl, with the two legs of the arch tied together, traced on horizontal 
glass during 33J hours. Movement of the bead of filament magnified 
about 26 times, and here reduied to one-half original scale. 

Before the above observations were made, some arched hypo- 
cotyls buried at the depth of a quarter of an inch were un- 
covered ; and in order to prevent the two legs of the arch 
from beginning to separate at once, they were tied together with 
fine silk. This was done partly because we wished to ascertain 
how long the hypocotyl, in its arched condition, would continue 
to move, and whether the movement when not masked and 
disturbed by the straightening process, indicated circumnu- 
tation. Firstly, a filament was fixed to the basal leg of an 
arched hypocotyl close above the summit of the radicle. The 
cotyledons were still partially enclosed within the seed-coats. 
The movement was traced (Fig. 4) from 9.20 a.m. on Dec. 



Chap. I. BRASSICA. 15 

23rd to 6.45 a.m. on Dec. 25th. No doubt the natural move- 
ment was much disturbed by the two legs having been tied 
together ; but we see that it was distinctly zigzag, first in one 
direction and then in an almost opposite one. After 3 p.m. on 
the 24th the arched hypocotyl sometimes remained stationary 
for a considerable time, and when moving, moved far slower than 
before. Therefore, on the morning of the 25th, the glass fila- 
ment was removed from the base of the basal leg, and was fixed 
horizontally on the summit of the arch, which, from the legs 
having been tied, had grown broad and almost flat. The 
movement was now traced during 23 hours (Fig. 5), and we 

Fig. 5. 




Brassica oleracea : circummtating movement of the crown of a buried and 
arched hypocotyl, with the two legs tied together, traced on a hori- 
zontal glass during 23 hours. Movement of the bead of the filament 
magnified about 58 time?, and here reduced to one-half original 



see that the course was still zigzag, which indicates a tendency 
to circumnutation. The base of the basal leg by this time had 
almost completely ceased to move. 

As soon as the cotyledons have been naturally dragged from 
beneath the ground, and the hypocotyl has straightened itself 
by growth along the inner or concave surface, there is nothing to 
interfere with the free movements of the parts ; and the circum- 
nutation now becomes much more regular and clearly displayed, 
as shown in the following cases: — A seedling was placed in 
front and near a north-east window with a line joining the 



16 CIECUMNUTATION OF SEEDLINGS. Chap. I. 

two cotyledons parallel to the window. It was thus left the 
whole day so as to accommodate itself to the light. On the 
following morning a filament was fixed to the midrib of the 
larger and taller cotyledon (which enfolds the other and smaller 
one, whilst still within the seed), and a mark being placed 
close behind, the movement of the whole plant, that is, of the 
hypocotyl and cotyledon, was traced greatly magnified on a ver- 
tical glass. At first the plant bent so much towards the light 
that it was useless to attempt to trace the movement ; but at 
10 a.m. heliotropism almost wholly ceased and the first dot was 

Fig. 6. 




Brassica olcracea; conjoint circumnutation of the hypocotyl and cotyledons 
during 10 hours 45 minutes. Figure here reduced to one-half original 
scale. 

made on the glass. The last was made at 8.45 p.m.; seventeen 
dots being altogether made in this interval of 10 h. 45 m. (see 
Fig. 6). It should be noticed that when I looked shortly after 
4 p.m. the bead was pointing off the glass, but it came on again 
at 5.30 p.m., and the course during this interval of 1 h. 30 m. has 
been filled up by imagination, but cannot be far from correct 
The bead moved seven times from side to side, and thus de- 
scribed 3s ellipses in 101 h.; each being completed on an 
average in 3 h. 4 m. 

On the previous day another seedling had been observed 
under similar conditions, excepting that the plant was so 



Chap. I. 



BKASS1CA. 



17 



placed that a line joining the two "cotyledons pointed towards 
the window ; and the filament was attached to the smaller coty- 
ledon on the side furthest from the window. Moreover, the 
plant was now for the first time placed in this position. Tho 
cotyledons bowed themselves greatly towards the light from 8 to 
10.50 a.m., when the first dot was made (Fig. 7). During the 

Fig. 7. 




Brassica oleracea : conjoint circumnutation of the hypocotyl and cotyledons, 
from 10.50 A.M. to 8 A.M. on the following morning. Tracing made 
on a vertical glass. 



next 12 hours the bead swept obliquely up and down 8 times 
and described 4 figures representing ellipses ; so that it travelled 
at nearly the same rate as in the previous case. During the 
night it moved upwards, owing to the sleep-movement of the 
cotyledons, and continued to move in the same direction till 
9 a.m. on the following morning ; but this latter movement 
would not have occurred with seedlings under their natural 
conditions fully exposed to the light. 
By 9.25 a.m. on this second day the same cotyledon had 



18 



CIKCUMNUTATION OF SEEDLINGS. Chap. 1 



begun to fall, and a dot was made on a fresh glass. The move- 
ment was traced until 5.30 p.m. as shown in (Fig. 8), which is 
given, because the course followed was much more irregular 

than on the two previous 



Fig. 8. 




Brassica oleracea : conjoint circumnutation 



occasions. During these 
8 hours the bead changed 
its course greatly 10 times. 
The upward movement of 
the cotyledon during the 
afternoon and early part 
of the night is here plainly 
shown. 

As the filaments were 
fixed in the three last 
cases to one of the coty- 
ledons, and as the hypo- ' 
cotyl was left free, the 
tracings show the move- 
ment of both organs con- 



of the hypocotyl and cotyledons during joined ; and we now 
8 hours. Figure here reduced to one- wished to ascertain whe- 
third of the original scale, as traced en a ther both circumriu tated. 
vertical glass. 

Filaments were therefore 

fixed horizontally to two hypocotyls close beneath the petioles 
of their cotyledons. These seedlings had stood for two days 
in the same position before a north-east window. In the morn- 
ing, up to about 11 a.m., they moved in zigzag lines towards 
the light; and at night they again became almost upright 
through apogeotropism. After about 11 a.m. they moved a 
little back from the light, often crossing and recrossing their 
former path in zigzag lines. The sky on this day varied much 
in brightness, and these observations merely proved that the 
hypocotyls were continually moving in a manner resembling 
circumnutation. On a previous day which was uniformly 
cloudy, a hypocotyl was firmly secured to a little stick, and 
a filament was fixed to the larger of the two cotyledons, and its 
movement was traced on a vertical glass. It fell greatly from 
8.52 a.m., when the first dot was made, till 10.55 a.m. ; it then rose 
greatly until 12.17 p.m. Afterwards it fell a little and made a 
loop, but by 2.22 p.m. it had risen a little and continued rising 
till 9.23 p.m., when it made another loop, and at 10.30 p.m. was 
again rising. These observations show that the cotyledons move 



Chap. I. 



BRASSICA. 



19 



vertically up and down all day long, and as there was some 
slight lateral movement, they circnmnutated. 

The cabbage was one of the first plants, the seedlings of which 
were observed by us, and we 9 

did not then know how far 
the circumnutation of the 
different parts was affected 
by light. Young seedlings 
were therefore kept in com- 
plete darkness except for a 
minute or two during each 
observation, when they were 
illuminated by a small wax 
taper held almost vertically 
above them. During the first 
day the hypocotyl of one 
changed its course 13 times 
(see Fig. 9) ; and it deserves 
notice that the longer axes 
of the figures described often 
cross one another at right or 
nearly right angles. Another 
seedling was observed in the 
same manner, but it was 
much older, for it had formed 
a true leaf a quarter of an 
inch in length, and the hy- 
pocotyl was If inch in height. 
The figure traced was a very 
complex one, though the 
movement was not so great 
in extent as in the last case. 
The hypocotyl of another 
seedling of the same age was 
secured to a little stick, and 
a filament having been fixed 
to the midrib of one of the 
cotyledons, the movement of 
the bead was traced during 14 h. 15 m. (see Fig. 10) in darkness. 
It should be noted that the chief movement of the cotyledons, 
namely, up and down, would be shown on a horizontal glass- 
plate only by the lines in the direction of the midrib (that is, 




Brassica oleracea : circumnutation of 
hypocotyl, in darkness, traced on a 
horizontal glass, by means of a fila- 
ment with a bead fixed across its 
summit, between 9.15 A.M. and 
8.30 a.m. on the following morn- 
ing. Figure here reduced to one- 
half of original scale. 



20 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 




up and down, as Fig. 10 here stands) being a little lengthened 
or shortened ; whereas any -lateral movement would be well 
exhibited. The present tracing shows 
that the cotyledon did thus move laterally 
(that is, from side to side in the tracing) 
12 times in the 14 h. 15 m. of observa- 
tion. Therefore the cotyledons certainly 
circumnutated, though the chief move- 
ment was up and down in a vertical 
plane. 

Bate of movement. — The movements of 
the hypocotyls and cotyledons of seedling 
cabbages of different ages have now been 
sufficiently illustrated. With respect to 
the rate, seedlings were placed under the 
microscope with the stage removed, and 
with a micrometer eye-piece so adjusted 
that each division equalled ^i_ inch ; the 
plants were illuminated by light passing 
through a solution of bichromate of potas- 
sium so as to eliminate heliotropism. 
Under these circumstances it was interest- 
ing to observe how rapidly the circum- 
nutating apex of a cotyledon passed across 
the divisions of the micrometer. Whilst 
travelling in any direction the apex generally oscillated back- 
wards and forwards to the extent of ^ and sometimes of nearly 
2I0 of an inch. These oscillations were quite different from the 
trembling caused by any disturbance in the same room or by 
the shutting of a distant door. The first seedling observed was 
nearly two inches in height and had been etiolated by having 
been grown in darkness. The tip of the cotyledon passed across 
10 divisions of the micrometer, that is, -^ of an inch, in 6 m. 
40 s. Short glass filaments were then fixed vertically to the 
hypocotyls of several seedlings so as to project a little above the 
cotyledons, thus exaggerating the rate of movement ; but only a 
few of the observations thus made are worth giving. The most 
remarkable fact was the oscillatory movement above described, 
and the difference of rate at which the point crossed the divi- 
sions of the micrometer, after short intervals of time. For 
instance, a tall not-etiolated seedling had been kept for 14 h. 
in darkness ; it was exposed before a north-east window for only 



h-assica oleracea : cir- 
cumnutation of a 
cotyledon, the hypo- 
cotyl having been 
secured to a stick, 
traoed on a horizon- 
tal glass, in dark- 
ness, from 8.15 A.M. 
to 10.30 P.M. Move- 
ment of the bead of 
the filament magni- 
fied 13 times. 



Ouap. I. GITHAGO. 21 

two or three minutes whilst a glass filament was fixed vertically 
to the hypocotyl ; it was then again placed in darkness for half 
an hour and afterwards observed by light passing through 
bichromate of potassium. The point, oscillating as usual, 
crossed five divisions of the micrometer (i. e. y^ inch) in 
1 m. 30 s. The seedling was then left in darkness for an hour, 
and now it required 3 m. 6 s. to cross one division, that is, 
15 m. 30 s. to have crossed five divisions. Another seedling, 
after being occasionally observed in the back part of a northern 
room with a very dull light, and left in complete darkness for 
intervals of half an hour, crossed five divisions in 5 m. in the 
direction of the window, so that we concluded that the move- 
ment was heliotropic. But this was probably not the case, for 
it was placed close to a north-east window and left there for 
25 m, after which time, instead of moving still more quickly 
towards the light, as might have been expected, it travelled 
only at the rate of 12 m. 30 s. for five divisions. It was then 
again left in complete darkness for lh., and the point now 
travelled in the same direction as before, but at the rate of 
3 m. 18 s. for five divisions. 

We shall have to recur to the cotyledons of the cabbage in a 
future chapter, when we treat of their sleep-movements. The 
circumnutation, also, of the leaves of fully-developed plants 
will hereafter be described. 

Fig. 11. 




Bithago segetum: circumnutation of hypocotyl, traced on a horizontal 
glass, by means of a filament fixed transversely across its summit, from 
8.15 A.M. to 12.15 P.M. on the following day. Movement of bead of 
filament magnified about 13 times, here reduced to one-half the original 
scale. 

Githago segetum (Caryophylleae).— A young seedling was dimly 
Uluminated from above, and the circumnutation of the hypo- 




22 CIKCUMNUTATION OF SEEDLINGS. CuAr. 1 

cotyl was observed during 28 h., as shown in Fig. 11. It moved 
in all directions ; the lines from right and to left in the figure 
being parallel to the blades of the cotyledons. The actual 
distance travelled from side to side by the summit of the 
hypocotyl was about "2 of an inch; but it was impossible to 
be accurate on this head, as the more obliquely the plant was 
viewed, after it had moved for some time, the more the distances 
were exaggerated. 

We endeavoured to observe the circumnutation of the coty- 
ledons, but as they close together unless kept exposed to a mode- 
rately bright light, and as the hypocotyl is extremely heliotropic, 
the necessary arrangements were too 
troublesome. We shall recur to the noc- 
turnal or sleep-movements of the cotyle- 
dons in a future chapter. 

Gossypium (var. Nankin cotton) (Mal- 
vaceae). — The circumnutation of a hypo- 
cotyl was observed in the hot-house, but 
Gossypium: arcumnu- the movement was so much exaggerated 

trac°ed on aTriSJ that the bead twice P aSSed for a time 0ut ° f 

tal glass, from 10.30 view. It was, however, manifest that two 

a.m. to 9.30 a.m. on somewhat irregular ellipses were nearly 

following morning, comp i et ed in 9 h. Another seedling, 

by means of a fila- .. , . r . , . , , , , , . 

ment fixed across *-* m - m beight, was then observed durmg 

its summit. Move- 23 h. ; but the observations were not 

ment of bead of fila- ma a e a t sufficiently short intervals, as 

TSLTSSSfSZ shOT ™ h ? the few dots in K s- 12 > and the 

minated from above, tracing was not now sufficiently enlarged. 

Nevertheless there could be no doubt 

about the circumnutation of the hypocotyl, which described 

in 12 h. a figure representing three irregular ellipses of unequal 

sizes. 

The cotyledons are in constant movement up and down during 
the whole day, and as they offer the unusual case of moving 
downwards late in the evening and in the early part of the 
night, many observations were made on them. A filament was 
fixed along the middle of one, and its movement traced on a 
vertical glass; but the tracing is not given, as the hypocotyl 
was not secured, so that it was impossible to distinguish clearly 
between its movement and that of the cotyledon. The coty- 
ledons rose from 10.30 a.m. to about 3 p.m. ; they then sank tiD 
10 p.m., rising, however, greatly in the latter part of the night 



Chap I. GOSSYriUM. 23 

The angles above the horizon at which the cotyledons of another 
seedling stood at different hours is recorded in the following 
6hort table : — ■ 

Oct. 20 2.50 p.m 25° above horizon. 

» 4.20 , 22° 

„ 5.20 „ 15° 

„ 10.40 „ 8 1 

Oct. 21 8.40 A.M 28° „ 

„ 11.15 „ 35° 

„ 9.11 r.M .. 10° below horizon. 

The position of the two cotyledons was roughly sketched at 
various hours with the same general result. 

In the following summer, the hypocotyl of a fourth seedling 
was secured to a little stick, and a glass filament with triangles 
of paper having been fixed to one of the cotyledons, its move- 
ments were traced on a vertical glass under a double skylight in 
the house. The first dot was made at 4.20 p.m. June 20th ; and 
the cotyledon fell till 10.15 p.m. in a nearly straight line. Just 
past midnight it was found a little lower and somewhat to one 
side. By the early morning, at 3.45 a.m., it had risen greatly, 
but by 6.20 a.m. had fallen a little. During the whole of this 
day (21st) it fell in a slightly zigzag line, but its normal course 
was disturbed by the want of sufficient illumination, for during 
the night it rose only a little, and travelled irregularly during 
the whole of the following day and night of June 22nd. The 
ascending and descending lines traced during the three days 
did not coincide, so that the movement was one of circumnuta- 
tion. This seedling was then taken back to the hot-house, and 
after five days was inspected at 10 p.m., when the cotyledons 
were found hanging so nearly vertically down, that they might 
justly be said to have been asleep. On the following morning 
they had resumed their usual horizontal position. 

(Jxalis rosea (Oxalideae). — The hypocotyl was secured to a little 
stick, and an extremely thin glass filament, with two triangles of 
paper, was attached to one of the cotyledons, which was *15 ir ch 
in length. In this and the following species the end of the 
petiole, where united to the blade, is developed into a pulvinus. 
The apex of the cotyledon stood only 5 inches from the vertical 
glass, so that its movement was not greatly exaggerated as loDg 
as it remained nearly horizontal ; but in the course of the day it 
both rose considerably above and fell beneath a horizontal posi- 
tion, and then of course the movement was much exaggerated. 



24 



CIECUMXUTATION OF SEEDLINGS. 



Chap. I. 



In Fig. 13 its course is shown from 6.45 a.m. on June 17th, to 

7.40 a.m. on the following morn- 
ing ; and we see that during the 
daytime, in the course of 11 h. 
15 m., it travelled thrice down 
and twice up. After 5.45 p.m. it 
moved rapidly downwards, and 
in an hour or two depended verti- 
cally ; it thus remained all night 
asleep. This position could not 
be represented on^ the vertical 
glass nor in the figure here given. 
By 6.40 a.m. on the following 
morning (18th) both cotyledons 
had risen greatly, and they con- 
tinued to rise until 8 a.m., when 
they stood almost horizontally. 
Their movement was traced dur- 
ing the whole of this day and 
until the next morning; but a 
tracing is not given, as it was 
closely similar to Eig. 13, except- 
ing that the lines were more 
zigzag. The cotyledons moved 
7 times, either upwards or down- 
wards; and at about 4 p.m. the 
great nocturnal sinking move- 
ment commenced. 

Another seedling was observed 
in a similar manner during nearly 
24 h., but with the difference that 
the hypocotyl was left free. The 
movement also was less magnified. 
Between 8.12 a.m. and 5 p.m. on 
the 18th, the apex of the cotyle- 
don moved 7 times upwards or 
downwards (Fig. 14). The noc- 
turnal sinking movement, which 
is merely a great increase of one 
of the diurnal oscillations, com- 
menced about 4 p.m. 
Oxales VaJdiviana.-— This species is interesting, as the coty- 




fi°30'a.m. 



U 



Oxalis rosea: circumnutation of 
cotyledons, the hypocotyl being 
secured to a stick ; illumina- 
ted from above. Figure here 
given one-halt of original scale. 



Chap. I. 



OXALIS. 



25 



ledons rise perpendicularly upwards at night so as to come into 
close contact, instead of sinking vertically downwards, as in the 
case of 0. rosea. A glass filament was fixed to a cotyledon, 
17 of an inch in length, and the hypocotyl was left free. On 



Fig. 14. 

8°12W. 
^ f 6°40'a.m.l9 f > 

i. 



Fig. 15. 



9*28' 



5°45' 



7°22 r p.fli. 




Oxalis rosea : conjoint circumnutation of 
the cotyledons and hypocotyl, traced 
from 8.12 a.m. on June 18th to 7.30 
A.M. 19th. The apex of the cotyledon 
stood only 3| inches from the vertical 
glass. Figure here given one-half of 
original scale. 



Oxalis Valdiviana : conjoint 
circumnutation of a cotyle- 
don and the hypocotyl, traced 
on vertical glass, during 24 
hours. Figure here given 
one-half of original scale ; 
seedling illuminated from 
above. 



the first day the seedling was placed too far from the vertical 
glass j S3 that the tracing was enormously exaggerated and tho 
movement could not be traced when the cotyledon either rose or 
sank much; but it was clearly seen that the cotyledons rose 
thrice and fell twice between 8.15 a.m. and 4.15 p.m. Early on 
Ihe following morning (June 19th) the apex of a cotyledon was 



26 CIRCUMNUTATION OF SEEDLINGS. Chap. 1 

placed only If inch from the vertical glass. At 6.40 a.m. it 
stood horizontally; it then fell till 8.35, and then rose. Al- 
together in the course of 12 h. it rose thrice and fell thrice, as 
may be seen in Fig. 15. The great nocturnal rise of the coty- 
ledons usually commences about 4 or 5 p.m., and on the following 
morning they are expanded or stand horizontally at about 6.30 
a.m. In the present instance, however, 1he great nocturnal rise 
did not commence till 7 p.m. ; but this was due to the hypocotyl 
having from some unknown cause temporarily bent to the left 
side, as is shown in the tracing. To ascertain positively that 
the hypocotyl circumnutated, a mark was placed at 8.15 p.m. 
behind the two now closed and vertical cotyledons ; and the 
movement of a glass filament fixed upright to the top of the 
hypocotyl was traced until 10.40 p.m. During this time it 
moved from side to side, as well as backwards and forwards, 
plainly showing circumnutation ; but the movement was small 
in extent. Therefore Fig. 15 represents fairly well the move- 
ments of the cotyledons alone, with the exception of the one 
great afternoon curvature to the left. 

Oxalis corniculata (var. cupred).— The cotyledons rise at night 
to a variable degree above the horizon, generally about 45° : 
those on some seedlings between 2 and 5 days old were found 
to be in continued movement all day long ; but the movements 
were more simple than in the last two species. This may have 
partly resulted from their not being sufficiently illuminated 
whilst being observed, as was shown by their not beginning tc 
rise until very late in the evening. 

Oxalis (Biophytum) sensitiva. — The cotyledons are highly re- 
markable from the amplitude and rapidity of their movements 
during the day. The angles at which they stood above or 
beneath the horizon were measured at short intervals of time ; 
and we regret that their course was not traced during the whole 
day. We will give only a few of the measurements, which were 
made whilst the seedlings were exposed to a temperature of 22 i° 
to 24s ° C. One cotyledon rose 70° in 11 m. : another, on a distinct 
seedling, fell 80° in 12 m. Immediately before this latter fall 
the same cotyledon had risen from a vertically downward to a 
vertically upward position in 1 h. 48 m., and had therefore passed 
through 180° in under 2 h. We have met with no other instance 
of a circumnutating movement of such great amplitude as 180° ; 
nor of such rapidity of movement as the passage through 80° in 
12 rn. The cotyledons of this plant sleep at night by rising 



Chap. I. 



TEOP^OLUM. 



27 



Fig. 16. 



vertically and coming into close contact. This upward move- 
ment differs from one of the great diurnal oscillations above 
described only by the position being permanent during the night 
and by its periodicity, as it always commences late in the 
evening. 

Tropxolum minus (?) (var. Tom Thumb) (Tropaeoleae). — The 
cotyledons are hypogean, or never rise above the ground. By 
removing the soil a buried epicotyl 
or plumule was found, with its 
Bummit arched abruptly down- 
wards, like the arched hypocotyl 
of the cabbage previously described. 
A glass filament with a bead at 
its end was affixed to the basal half 
or leg, just above the hypogean 
cotyledons, which were again almost 
surrounded by loose earth. The 
tracing (Fig. 16) shows the course 
of the bead during 11 h. After the 
last dot given in the figure, the 
bead moved to a great distance, 
and finally off the glass, in the 
direction indicated by the broken 
line. This great movement, due to 
increased growth along the con- 
cave surface of the arch, was caused 
by the basal leg bending back- 
wards from the upper part, that is 
in a direction opposite to the depen- 
dent tip, in the same manner as 
occurred with the hypocotyl of 
the cabbage. Another buried and 

arched epicotyl was observed in the same manner, excepting 
that the two legs of the arch were tied together with fine silk 
for the sake of preventing the great movement just mentioned. 
It moved, however, in the evening in the same direction as 
before, but the line followed was not so straight. During the 
morning the tied arch moved in an irregularly circular, strongly 
zigzag course, and to a greater distance than in the previous 
case, as was shown in a tracing, magnified 18 times. The move- 
ments of a young plant bearing a few leaves and of a mature 
plant, will hereafter be described. 




Tropceolum minus (?) : circum- 
nutation of buried and arched 
epicotyl, traced on a horizon- 
tal glass, from 9.20 a.m. to 
8. 15 P.M. Movement of bead 
of filament magnified 27 
times. 



28 



CIRCUMNUTATION OF SEEDLINGS. Chap, t 



Citrus aurantinm (Orange) (Aurantiacese). — The cotyledons 
are hypogean. The circumnutation of an epicotyl, which at the 
close of our observations was "59 of an inch (15 mm.) in height 
above the ground, is shown in the annexed figure (Fig. 17), as 
observed during a period of 44 h. 40 m. 



Fie. 17. 




Citrus aurantium: circumnutation of epicotyl with a filament fixed traus* 
versely near its apex, traced on a horizontal glass, from 12.13 P.M. on 
Feb. 20th to 8.55 A.M. on 22nd. The movement of the bead of the 
filament was at first magnified 21 times, or 10^, in figure here given, 
and afterwards 36 times, or 18 as here given ; seedling illuminated 
from above. 



JEsculus hippocastanum (Hippocastaneae). — Germinating seeds 
were placed in a tin box, kept moist internally, with a sloping 
bank of damp argillaceous sand, on which four smoked glass- 
plates rested, inclined at angles of 70° and 65° with the 
horizon. The tips of the radicles were placed so as just to 
touch the upper end of the glass-plates, and, as they grew 
downwards they pressed lightly, owing to geotropism, on the 
smoked surfaces, and left tracks of their course. In the middle 
part of each track the glass was swept clean, but the margins 
were much blurred and irregular. Copies of two of these tracks 
(all four being nearly alike) were made on tracing paper placed 
over the glass-plates after they had been varnished ; and they 
are as exact as possible, considering the nature of the margins 
(Fig. 18). They suffice to show that there was some lateral, 
almost serpentine movement, and that the tips in their down- 
ward course pressed with unequal force on the plates, as 



Chap. I 



VICIA. 



29 



Fig. 18. 




JEsculus h'ppocastanum : out« 
lines of tracks left on in- 
clined glass-plates by tips 
of radicles. In A the plate 
was inclined at 70° with 
the horizon, and the radicle 
was 1 * 9 inch in length, and 
•23 inch in diameter at base. 
In B the plate was inclined 
65° with the horizon, and 



the radicle 
larger. 



was a trifle 



the tracks varied in breadth. The more perfectly serpentine 
tracks made by the radicles of Phaseolus multijlorus and Vicia 
faba (presently to be described), render 
it almost certain that the radicles of 
the present plant circumnutated. 

Phaseolus multijlorus (Leguminosse). 
—Four smoked glass-plates were ar- 
ranged in the same manner as des- 
cribed under iEsculus, and the tracks 
left by the tips of fonr radicles of the 
present plant, whilst growing down- 
wards, were photographed as trans- 
parent objects. Three of them are 
here exactly copied (Fig. 19). Their 
serpentine courses show that the tips 
moved regularly from side to side; 
they also pressed alternately with 
greater or less force on the plates, 
sometimes rising up and leaving them 
altogether for a very short distance ; 
but this was better seen on the 
original plates than in the copies. 

These radicles therefore were continually moving in all direc- 
tions — that is, they circumnutated. The distance between the 
extreme right and left positions 
of the radicle A, in its lateral 
movement, was 2 mm., as ascer- 
tained by measurement with an 
eye-piece micrometer. 

Vicia faba (Common Bean) 
(Leguminosse). — Radicle. — Some 
beans were allowed to germinate 
on bare sand, and after one had 
protruded its radicle to a length 
of *2 of an inch, it was turned 
npside down, so that the radicle, 
which was kept in damp air, 
now stood upright. A filament, 
nearly an inch in length, was 
affixed obliquely near its tip; and the movement of the 
terminal bead was traced from 8.30 a.m. to 10.30 p.m., as shown 
in Fig. 18. The radicle at first changed its course twice 



Fig. 19. 



i 



Phaseolus multijlorus : tracks left 
on inclined smoked glass-plates 
by tips of radicles in growing 
downwards. A and C, plates 
inclined at 60°, B inclined at 
68° with the horizon. 



30 



CIRCUMNUTATION OF SEEDLINGS. Chap. 1 



abruptly, then made a small loop and then a larger zigzag 
curve. During the night arid till 11a.m. on the following 

Fig. 20. 




Vicia faba: circumnutation of a radicle, at first pointing vertically up- 
wards, kept in darkness, traced on a horizontal glass, during 14 hour.*. 
Movement of bead of filament magnified 23 times, here reduced to 
one-half of original scale. 



morning, the head moved to a great distance in a nearly straight 
line, in the direction indicated by the broken line in the figure. 
This resulted from the tip bending quickly downwards, as it 
had now become much declined, and had thus gained a position 
highly favourable for the action of geotropism. 

Fig. 21. 




0. D. E. 

ficia faba: tracks left on inclined smoked glass-plates, by tips of radicles 
in growing downwards. Plate C was inclined at 63°, plates A and D 
at 71°, plate B at 75°, and plate E at a few degrees beneath the 
horizon. 



Chap. I. VICIA. 31 

We next experimented on nearly a score of radicles by allowing 
them to grow downwards over inclined plates of smoked glass, 
in exactly the same manner as with .ZEsculus and Phaseolus. 
Some of the plates were inclined only a few degrees beneath 
the horizon, but most of them between 60° and 75°. In the 
latter cases the radicles in growing downwards were deflected 
only a little from the direction which they had followed whilst 
germinating in sawdust, and they pressed lightly on the glass~ 
plates (Fig. 21). Five of the most distinct tracks are here 
copied, and they are all slightly sinuous, showing circumnuta- 
tion. Moreover, a close examination of almost every one of the 
tracks clearly showed that the tips in their downward course 
had alternately pressed with greater or less force on the plates, 
and had sometimes risen up so as nearly to leave them for short 
intervals. The distance between the extreme right and left 
positions of the radicle A was 0*7 mm., ascertained in the same 
manner as in the case of Phaseolus. 

Epicotyl. — At the point where the radicle had protruded from 
a bean laid on its side, a flattened solid lump projected '1 of an 
inch, in the same horizontal plane with the bean. This protuber- 
ance consisted of the convex summit of the arched epicotyl; 
and as it became developed the two legs of the arch curved 
themselves laterally upwards, owing to apogeotropism, at such 
a rate that the arch stood highly inclined after 14 h., and 
vertically in 48 h. A filament was fixed to the crown of 
the protuberance before any arch was visible, but the basal 
half grew so quickly that on the second morning the end of the 
filament was bowed greatly downwards. It was therefore re- 
moved and fixed lower down. The line traced during these two 
days extended in the same general direction, and was in parts 
nearly straight, and in others plainly zigzag, thus giving some 
evidence of circumnutation. 

As the arched epicotyl, in whatever position it may be placed, 
bends quickly upwards through apogeotropism, and as the two 
legs tend at a very early age to separate from one another, as 
soon as they are relieved from the pressure of the surrounding 
earth, it was difficult to ascertain positively whether the epicotyl, 
whilst remaining arched, circumnutated. Therefore some rather 
deeply buried beans were uncovered, and the two legs of the 
arches were tied together, as had been done with the epicotyl 
of Tropaeolum and the hypocotyl of the Cabbage. The move- 
ments of the tied arches were traced in the usual manner on 



32 CIRCUMNUTATION OF SEEDLINGS. Chap. L 

two occasions during three days. But the tracings made under 
such unnatural conditions are not worth giving ; and it need 
only be said that the lines were decidedly zigzag, and that 
small loops were occasionally formed. We may therefore con- 
clude that the epicotyl circumnutates whilst still arched and 
before it has grown tall enough to break through the surface 
of the ground. 

In order to observe the movements of the epicotyl at a some- 
what more advanced age, a filament was fixed near the base of 
one which was no longer arched, for its upper half now formed 
a right angle with the lower half. This bean had germinated 
on bare damp sand, and the epicotyl began to straighten itself 
much sooner than would have occurred if it had been properly 
planted. The course pursued during 50 h. (from 9 a.m. Dec. 
26th, to 11 a.m. 28th) is here shown (Fig. 22) ; and we see 

Fig. 22. 




Vicia faba: circumnutation of young epicotyl, traced in darkness during 
50 hours on a horizontal glass. Movement of bead of filament mag- 
nified 20 times, here reduced to one-half of original scale. 

that the epicotyl circumnutated during the whole time. Its 
basal part grew so much during the 50 h. that the filament 
at the end of our observations was attached at the height of 
"4 inch above the upper surface of the bean, instead of close 
to it. If the bean had been properly planttd, this part of the 
epicotyl would still have been beneath the soil. 

Late in the evening of the 28th, some hours after the above 
observations were completed, the epicotyl had grown much 
straighter, for the upper part now formed a widely open angle 
with the lower part. A filament was fixed to the upright basal 
part, higher up than before, close beneath the lowest scale-like 
process or homologue of a leaf; and its movement was traced 



Chap. I. 



LATHYRUS. 



33 



during 38 h. (Fig. 23). We here again have plain evidence of 
continued circunmutation. Had the bean been properly planted, 
the part of the epicotyl to which the filament was attached, the 

Fig. 23. 




Vicia faba : circumnutation of the same epicotyl as in Fig. 22, a little more 
Advanced in age, traced under similar conditions as before, from 8.40 A.M. 
Dec. 28th, to 10.50 A.M. 30th. Movement of bead here magnified 
20 times. 

movement of which is here shown, would probably have just 
risen above the surface of the ground. 

Lathyrus nissolia (Leguminosse). — This plant was selected for 
observation from being an abnormal form with grass-like leaves. 

Fig. 24. 




Lathyrus n'ssolia: circumnutation of stem of young seedling, traced in 
darkness on a horizontal glass, from 6.45 A.M. Nov. 22nd, to 7 a.m. 
23rd. Movement of end of leaf magnified about 12 times, here re- 
duced to one-half of original scale. 

The cotyledons are hypogean, and the epicotyl breaks through 
the ground in an arched form. The movements of a stem, 12 
inch in height, consisting of three internodes, the lower one 
almost wholly subterranean, and the upper one bearing a short, 



34 



CIKCUMNUTATION OF SEEDLINGS. 



Chap. I. 



narrow leaf, is shown during 2i h., in Fig. 24. No glass filament 
was employed, but a mark was placed beneath the apex of the 
leaf. The actual length of the longer of the two ellipses de- 
scribed by the stem was about *14 of an inch. On the previous 
day the chief line of movement was nearly at right angles to 
that shown in the present figure, and it was more simple. 
Cassia tora* (Leguminosae). — A seedling was placed before a 



Fig. 25 
ram. riO>a.m.B& 




9°a.m 



Cteril tora : conjoint circumnutation of cotyledons and hypocotyl, traced 
on vertical glass, from 7.10 a.m. Sept. 25th to 7.30 a.m. 26th. Figure 
here given reduced to one-half of original scale. 



* Seeds of tl.is plant, which 
grew near the sea-side, were sent 
to us by Fritz Miiller from S. 
Brazil. The seedlings did not 



flourish or flower well with us; 
they were sent to Kew, and were 
pronounced not to be distinguish- 
able from C. tora. 



Ohap. I. LOTUS. 35 

north-east window ; it bent very little towards it, as the bypo- 
cotyl which was left free was rather old, and therefore not highly 
heliotropic. A filament had been fixed to the midrib of one of 
the cotyledons, and the movement of the whole seedling was 
traced during two days. The circumnutation of the hypocotyl 
is quite insignificant compared with that of the cotyledons. 
These rise up vertically at night and come into close contact ; so 
that they may be said to sleep. This seedling was so old that a 
very small true leaf had been developed, which at night was 
completely hidden by the closed cotyledons. On Sept. 24th, 
between 8 a.m. and 5 p.m., the cotyledons moved five times up 
and five times down; they therefore described five irregular 
ellipses in the course of the 9 h. The great nocturnal rise com- 
menced about 4.30 p.m. 

On the following morning (Sept. 25th) the movement of 
the same cotyledon was again traced in the same manner 
during 24 h. ; and a copy of the tracing is here given (Fig. 25). 
The morning was cold, and the window had been accidentally 
left open for a short time, which must have chilled the plant ; 
and this probably prevented it from moving quite as freely as 
on the previous day ; for it rose only four and sank only four 
times during the day, one of the oscillations being very small. 
At 7.10 a.m., when the first dot was made, the cotyledons were 
not fully open or awake ; they continued to open till about 9 a.m., 
by which time they had sunk a little beneath the horizon : by 
9.30 a.m. they had risen, and then they oscillated up and down ; 
but the upward and downward lines never quite coincided. At 
about 4.30 p.m. the great nocturnal rise commenced. At 7 a.m. 
on the following morning (Sept. 26th) they occupied nearly 
the same level as on the previous morning, as shown in the 
diagram : they then began to open or sink in the usual manner. 
The diagram leads to the belief that the great periodical daily 
rise and fall does not differ essentially, excepting in amplitude, 
from the oscillations during the middle of the day. 

Lotus Jacobosus (Leguminosse). — The cotyledons of this plant, 
after the few first days of their life, rise so as to stand almost, 
though rarely quite, vertically at night. They continue to act i'n 
this manner for a long time even after the development of some 
of the true leaves. With seedlings, 3 inches in height, and bear- 
ing five or six leaves, they roso at night about 45°. They con- 
tinued to act thus for about an additional fortnight. Subse- 
quently they remained horizontal at Dight, though still green, 



36 CIKCUMNUTATICXN" OF SEEDLINGS. Chap. 1. 

and at last dropped off. Their rising at night so as to stand 
almost vertically appears to depend largely on temperature; 
for when the seedlings were kept in a cool house, though they 
still continued to grow, the cotyledons did not become vertical 
at night. It is remarkable that the cotyledons do not generally 
rise at night to any conspicuous extent during the first four or 
five days after germination ; but the period was extremely 
variable with seedlings kept under the same conditions; and 
many were observed. Glass filaments with minute triangles of 
paper were fixed to the cotyledons (1£ mm. in breadth) of two 
seedlings, only 24 h. old, and the hypocotyl was secured to a 
stick; their movements greatly magnified were traced, and they 
certainly circumnutated the whole time on a small scale, but 
they did not exhibit any distinct nocturnal and diurnal move- 
ment. The hypocotyls, when left free, circumnutated over a 
large space. 

Another and much older seedling, bearing a half-developed 
leaf, had its movements traced in a similar manner during the 
three first days and nights of June ; but seedlings at this age 
appear to be very sensitive to a deficiency of light ; they were 
observed under a rather dim skylight, at a temperature of 
between 16° to 17 £° C. ; and apparently, in consequence of these 
conditions, the great daily movement of the cotyledons ceased 
on the third day. During the first two days they began rising 
in the early afternoon in a nearly straight line, until between 
6 and 7 p.m., when they stood vertically. During the latter 
part of the night, or more probably in the early morning, they 
began to fall or open, so that by 6.45 a.m. they stood fully 
expanded and horizontal. They continued to fall slowly for 
some time, and during the second day described a single 
small ellipse, between 9 a.m. and 2 p.m., in addition to the 
great diurnal movement. The course pursued during the 
whole 24 h. was far less complex than in the foregoing case of 
Cassia. On the third morning they fell very much, and then 
circumnutated on a small scale round the same spot ; by 8.20 
r.M. they showed no tendency to rise at night. Nor did the 
cotyledons of any of the many other seedlings in the same pot 
rise ; and so it was on the following night of June 5th. The 
pot was then taken back into the hot-house, where it was exposed 
to the sun, and on the succeeding night all the cotyledons rose 
again to a high angle, but did not stand quite vertically. On 
each of the above days the line representing the great nocturnal 



Chap. L CYTISUS. 37 

rise did not coincide with that of the great diurnal fall, so that 
narrow ellipses were described, as is the usual rule with circum- 
nutating organs. The cotyledons are provided with a pulvinus, 
and its development will hereafter be described. 

Mimosa pudica (Leguminosse). — The cotyledons rise up verti- 
cally at night, so as to close together. Two seedlings were 
observed in the greenhouse (temp. 16° to 17° C. or 63° to 65° F.). 
Their hypocotyls were secured to sticks, and glass filaments 
bearing little triangles of paper were affixed to the cotyledons of 
both. Their movements were traced on a vertical glass during 
24 h. on November 13th. The pot had stood for some time iti 
the same position, and they were chiefly illuminated through 
the glass-roof. The cotyledons of one of these seedlings moved 
downward in the morning till 11.30 a.m., and then rose, moving 
rapidly in the evening until they stood vertically, so that in this 
case there was simply a single great daily fall and rise. The 
other seedling behaved rather differently, for it fell in the morn- 
ing until 11.30 a.m., and then rose, but after 12.10 p.m. again fell ; 
and the great evening rise did not begin until 1.22 p.m. On the 
following morning this cotyledon had fallen greatly from its 
vertical position by 8.15 a.m. Two other seedlings (one seven 
and the other eight days old) had been previously observed 
under unfavourable circumstances, for they had been brought 
into a room and placed before a north-east window, where the 
temperature was between only 56° and 57° F. They had, more- 
over, to be protected from lateral light, and perhaps were not 
sufficiently illuminated. Under these circumstances the coty- 
ledons moved simply downwards from 7 a.m. till 2 p.m., after 
which hour and during a large part of the night they con- 
tinued to rise. Between 7 and 8 a.m. on the following morning 
they fell again ; but on this second and likewise on the third 
day the movements became irregular, and between 3 and 10.30 
p.m. they circumnutated to a small extent about the same spot ; 
but they did not rise at night. Nevertheless, on the following 
night they rose as usual. 

Cytisusfra grans (Leguminosae). — Only a few observations were 
made on this plant. The hypocotyl circumnutated to a con- 
siderable extent, but in a simple manner — namely, for two hours 
in one direction, and then much more slowly back again in 
a zigzag course, almost parallel to the first line, and beyond the 
starting-point. It moved in the same direction all night, but 
next morning began to return. The cotyledons continually 



38 CIKCUMNUTATION OF SEEDLINGS. Cbap. 1. 

move both up and down and laterally ; but they do not rise up 
at night in a conspicuous manner. 

Lupinus luteus (Leguminosse). — Seedlings of this plant were 
observed because the cotyledons are so thick (about 08 of an 
inch) that it seemed unlikely that they would move. Our 
observations were not very successful, as the seedlings are 
strongly heliotropic, and their circumnutation could not be 
accurately observed near a north-east window, although they 
had been kept during the previous day in the same position. 
A seedling was then placed in darkness with the hypocotyl 
secured to a stick; both cotyledons rose a little at first, and 
then fell during the rest of the day; in the evening between 
5 and 6 p.m. they moved very slowly ; during the night one 
continued to fall and the other rose, though only a little. The 
tracing was not much magnified, and as the lines were plainly 
zigzag, the cotyledons must have moved a little laterally, that 
is, they must have circumnutated. 

The hypocotyl is rather thick, about * 12 of inch ; nevertheless 
it circumnutated in a complex course, though to a small extent. 
The movement of an old seedling with two true leaves partially 
developed, was observed in the daik. As the movement was 
magnified about 100 times it is not trustworthy and is not 
given ; but there could be no doubt that the hypocotyl moved 
in all directions during the day, changing its course 19 times. 
The extreme actual distance from side to side through which 
the upper part of the hypocotyl passed in the course of 14g hours 
was only fa of an inch ; it sometimes travelled at the rate of 
fa of an inch in an hour. 

Cucurbita ovifera (Cucurbitaceae). — Radicle : a seed which had 

Fig. 26. 



A- 



Cucurbita ovf era: course followed by a radicle in bending geotropicnlly 
downwards, traced on a horizontal glass, between 11.25 A.M. and 10.25 
P.M. ; the direction during the night is indicated by the broken line. 
Movement of bead magnified 14 times. 

germinated on damp sand was fixed so that the slightly curved 
radicle, which was only - 07 inch in length, stood almost vertically 



Char I. 



CUCUKBITA. 



39 



upwards, in which position geotropism would act at first with 
little power. A filament was attached near to its base, and 
projected at about an angle of -15° above the horizon. The 
general course followed during the 11 hours of observation and 
during the following night, is shown in the accompanying 
diagram (Fig. 26), and was plainly due to geotropism ; but it 
was also clear that the radicle circumnutated. By the next 
morning the tip had curved so much downwards that the fila- 
ment, instead of projecting at 45>° above the horizon, was nearly 
horizontal. Another germinating seed was turned upside down 
and covered with damp sand ; and a filament was fastened to 
the radicle so as to project at an angle of about 50° above the 
horizon; this radicle was '35 of an inch in length and a littlo 
curved. The course pursued was mainly governed, as in the 
last case, by geotropism, but the line traced during 12 hours and 
magnified as before was more strongly zigzag, again showing 
circumnutation. 

Four radicles were allowed to grow downwards over plates 
of smoked glass, inclined at 70° to the horizon, under the 



Fig. 27. 



A. B. 

Cucurbita ovifera: tracks 
left by tips of radicles 
in growing downwards 
over smoked glass- 
plates, inclined at 70° 
to the horizon. 



C 



Fig. 28. 



--<-- ' 




J 



Cucurbita ovifcra : circumnuta- 
tion of arched hypocotyl at 
a very early age, traced in 
darkness on a horizontal glass, 
from 8 A.M. to 10.20 a.m. on 
the following day. The move- 
ment of the bead magnified 
20 times, here reduced to one- 
half of original scale. 



same conditions as in the cases of JEsculus, Phaseolus, and 
Vicia. Facsimiles are here given (Fig. 27) of two of these 
tracbs ; and a third short one was almost as plainly serpentine 
as that at A. It was also manifest by a greater or less amount 
of soot having been swept off the glasses, that the tips had 
4 



w 



CIRCUMXUTATION OF SEEDLINGS. 



Chap, t 



Fie;. 29. 



pressed alternately with greater and less force on them. There 
must, therefore, havo 1™en movement in at least two planes at 
right angles to one another. These radicles were so delicate that 
they rarely had the power to sweep the glasses quite clean. One 
of them had developed some lateral or secondary rootlets, which 
projected a few degrees beneath the horizon ; and it is an im- 
portant fact that three of them left distinctly serpentine tracks 
on the smoked surface, showing beyond doubt that they had 
circumnutated like the main or primary radicle. But the 
tracks were so slight that they could not be traced and copied 
after the smoked surface had been varnished. 

Hypocotyl. — A seed lying on damp sand was firmly fixed by 
two crossed wires and by its own growing radicle. The cotyle- 
dons were still enclosed within the seed-coats; and the short 

hypocotyl, between the summit of 
the radicle and the cotyledons, 
was as yet only slightly arched. A 
filament (-85 of inch in length) 
was attached at an angle of 35° 
above the horizon to the side of 
the arch adjoining the cotyle- 
dons. This part would ultimately 
form the upper end of the hypo- 
cotyl, after it had grown straight 
and vertical. Had the seed been 
properly planted, the hypocotyl at 
this stage of growth would have 
been deeply buried beneath the 
surface. The course followed by 
the bead of the filament is shown 
in Fig. 28. The chief lines of 
movement from left to right in the 
figure were parallel to the plane 
of the two united cotyledons and 
of the flattened seed; ard this 
movement would aid in dragging 
them out of the seed-coats, which 
are held down by a special struc- 
ture hereafter to be described. The movement at right angles 
to the above lines was due to the arched hypocotyl becoming 
more arched as it increased in height, The foregoing observa- 
tions apply to the leg of the arch next to the cotyledons, but 




Cucurbit a oti'j 



circumnuta- 



tion of straight and verti- 
cal hypocotyl, with filament 
fastened transversely across 
its upper end, traced in dark- 
ness on a horizontal glass, 
from 8.30 A.M. to 8.30 p.m. 
The movement of the terminal 
bead originally magnified 
about 18 times, here only 4^ 
times. 



Uhap. I. CUCUEBITA. 41 

the other leg adjoining the radicle likewise circumnutated at on 
equally early age. 

The movement of the same hypocotyl after it had become 
straight and vertical, but with the cotyledons only partially 
expanded, is shown in Fig. 29. The course pursued during 12 h. 
apparently represents four and a half ellipses or ovals, with 
the longer axis of the first at nearly right angles to that of the 
others. The longer axes of all were oblique to a line joining 
the opposite cotyledons. The actual, extreme distance from 
side to side over which the summit of the tall hypocotyl 
passed in the course of 12 h. was *28 of an inch. The original 
figure was traced on a large scale, and from the obliquity of 
the line of view the outer parts of the diagram are much 
exaggerated. 

Cotyledons. — On two occasions the movements of the cotyle- 
dons were traced on a vertical glass, and as the ascending and 
descending lines did not quite coincide, very narrow ellipses 
were formed; they therefore circumnutated. "Whilst young 
they rise vertically up at night, but their tips always remain 
reflexed ; on the following morning they sink down again. With 
a seedling kept in complete darkness they moved in. the same 
manner, for they sank from 8745 a.m. to 4.30 p.m. ; they then 
began to rise and remained close together until 10 p.m., when 
they were last observed. At 7 a.m. on the following morning 
they were as much expanded as at any hour on the previous 
day. The cotyledons of another young seedling, exposed to the 
light, were fully open for the first time on a certain day, but 
were found completely closed at 7 a.m. on the following morning. 
They soon began to expand again, and continued doing so till 
about 5 p.m. ; they then began to rise, and by 10.30 p.m. stood 
vortically and were almost closed. At 7 a.m. on. the third morn- 
ing they were nearly vertical, and again expanded during the 
day; on the fourth morning they were not closed, yet they 
opened a little in the course of the day and rose a little on the 
following night. By this time a minute true leaf had become 
developed. Another seedling, still older, bearing a well-developed 
leaf, had a sharp rigid filament affixed to one of its cotyledons 
(85 mm. in length), which recorded its own movements on 
a revolving drum with smoked paper. The observations were 
made in the hot-house, where the plant -had lived, so that there 
was no change in temperature or light. The record commenced 
at 11 a.m. on February 18th; and fioni this hour till 3 p.m. the 



£2 CIKCUMNUTATION OF SEEDLINGS. Cha*. J, 

cotyledon fell; it then rose rapidly till 9 p.m., then verj 
gradually till 3 a.m. February 19th,. after which hour it sank 
gradually till 4.30 p.m. ; but the downward movement was inter- 
rupted by one slight rise or oscillation about 1.30 p.m. Aftei 
4.30 p.m. (19th) the cotyledon rose till 1 a.m. (in the night of 
February 20th) and then sank very gradually till 9.30 a.m., 
when our observations ceased. The amount of movement was 
greater on the 18th than on the 19th or on the morning of 
the 20th. 

Cucurbita aurardia. — An arched hypocotyl was found buried a 
little beneath the surface of the soil ; and in order to prevent it 
straightening itself quickly, when relieved from the surrounding- 
pressure of the soil, the two legs of the arch were tied together. 
The seed was then lightly covered with loose damp earth. A 
filament with a bead at the end was affixed to the basal leg, the 
movements of which were observed during two days in the 
usual manner. On the first day the arch moved in a zigzag line 
towards the side of the basal leg. On the next day, by which 
time the dependent cotyledons had been dragged above the sur- 
face of the soil, the tied arch changed its course greatly nine 
times in the course of 14J h. It swept a large, extremely irre- 
gular, circular figure, returning at night to nearly the same 
spot whence it had started early in the morning. The line was 
so strongly zigzag that it apparently represented five ellipses, with 
their longer axes pointing in various directions. With respect 
to the periodical movements of the cotyledons, those of several 
young seedlings formed together at 4 p.m. an angle of about 60°, 
and at 10 p.m. their lower parts stood vertically and were in 
contact ; their tips, however, as is usual in the genus, were per- 
manently reflexed. These cotyledons, at 7 a.m. on the following 
morning, were again well expanded. 

Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbi- 
taceae). — A seedling opened its cotyledons, the movements of 
which were alone observed, slightly on June 27th, and closed 
them at night: next day, at noon (28th), they included an 
angle of 53°, and at 10 p.m. they were in close contact, so that 
each had risen 26 h°. At noon, on the 29th, they included an 
angle of 118°, and at 10 p.m. an angle of 54°, so each had 
risen 32°. On the following day they were still more open, and 
the nocturnal rise was greater, but the angles were not measured. 
Two other seedlings were observed, and behaved during thre6 
days in a closely similar manner. The cotyledons, therefore. 



Uhap. J. 



CUCUKLITA. 



43 



Fig. 



10° 95 p.m. 
12th ' 



G"38 r a.m.\ 
13^ 



open more and more on each succeeding day, and rise each 
night about 30° ; consequently during the first two nights of 
their life they stand vertically and 
come into contact. 

In order to ascertain more ac- 
curately the nature of these move- 
ments, the hypocotyl of a seedling, 
with its cotyledons well expanded, 
was secured to a little stick, and a 
filament with triangles of paper 
was affixed to one of the cotyledons. 
The observations were made under 
a rather dim skylight, and the 
temperature during the whole time 
was between 17£° to 18° C. (63° to 
65° F.), Had the temperature been 
higher and the light brighter, the 
movements would probably have 
been greater. On July 13 th (see 
Fig. 30), the cotyledon fell from 
7.35 a.m. till 10 a.m. ; it then rose 
(rapidly after 4 p.m.) till it stood 
quite vertically at 8.40 r.M. During 
the early morning of the next day 
(12th) it fell, and continued to fall 
till 8 a.m., after which hour it rose, 
then fell, and again rose, so that by 
10.35 p.m. it stood much higher than 
it did in the morning, but was not 
vertical as on the preceding night. 
During the following early morn- 
ing and whole day (13th) it fell and 
circumnutated, but had not risen 
when observed late in the evening ; 
and this was probably due to the 
deficiency of heat or light, or of 
both. We thus see that the coty- 
ledons became more widely open at 



,7°35'} 



.10° a. m* 



10° 3 0rp.i 



Vl 14t/i 

Lagcnaria vulgaris: circumnu- 
tation of a cotyledon, 1§ 
inch in length, apex only 4f 
inches from the vertical glass, 
on which its movements were 
traced from 7.35 a.m. July 
11th to 9.5 A.M. on the 14th. 
Figure here given reduced 
to one-third of original scale. 



noon on each succeeding day ; and 

that they rose considerably each night, though not acquiring 

a vertical position, except during the first two nights. 

(Jucumis dudaim (Cucurbitaceae). — Two seedlings had opened 



34 



CIRCUMNUTATION OF SEEDLINGS. 



Chap. I. 



their cotyledons for the first time during the day, — one to the 
extent of 90° and the other rather more; they remained in 
nearly the same position until 10.40 p.m. ; but by 7 a.m. on the 
following morning the one which had been previously open to 
the extent of 90° had its cotyledons vertical and completely 
shut ; the other seedling had them nearly shut. Later in the 
morning they opened in the ordinary manner. It appears 
therefore that the cotyledons of this plant close and open at 
somewhat different periods from those of the foregoing species 
of the allied genera of Cucurbita and Lagenaria. 

Opuntia basilaris (Cactese). — A seedling was carefully ob- 
served, because considering its 
appearance and the nature of the 
mature plant, it seemed very un- 
likely that either the hypocotyl or 
cotyledons would circumnutate to 
an appreciable extent. The coty- 
ledons were well developed, being 
•9 of an inch in length, "22 in 
breadth, and '15 in thickness. 
The almost cylindrical hypocotyl, 
now bearing a minute spinous bud 
on its summit, was only "45 of an 
inch in height, and "19 in dia- 
meter. The tracing (Fig. 31) shows 
the combined movement of the 



Fig. 31. 




I 



Opuntia basilaris : conjoint cir- 
cumnutation of hypocotyl 
and cotyledon ; filament hypocotyl and of one of the COty- 
fixed longitudinally to coty- ^ {mm 44& Qn M 2gth 

ledon, and movement traced ' J 

during 66 h. on horizontal to 11 A.M. on the 31st. On the 29th 
glass. Movement of the ter- a nearly perfect ellipse was com- 
pleted. On the 30th the hypocotyl 
moved, from some unknown cause, 



minal bead magnified about 
30 times, here reduced to one- 
third scale. Seedling kept in 
hot-house, feebly illuminated in the same general direction in a 
from above. zigzag line ; but between 4.30 and 

10 p.m. almost completed a second 
small ellipse. The cotyledons move only a little up and down : 
thus at 10.15 p.m. they stood only 10° higher than at noon. The 
chief seat of movement therefore, at least when the cotyledons 
are rather old as in the present case, lies in the hypocotyl. The 
ellipse described on the 29th had its longer axis directed at 
nearly right angles to a line joining the two cotyledons. The 
actual amount of movement cf the bead at the end of the 



Uhak L 



PRIMULA. 



45 




filament was, as far as could be ascertained, about "14 of an 
inch. 

lldianthus annuus (Composite). — The upper part of the 
hypocotyl moved during the 

day-time in the course I r ig- 32 - 

shown in the annexed figure 
(Fig. 32). As the line runs 
in various directions, cross- 
ing itself several times, 
the movement may be con- 
sidered as one of circumnu- 
tation. The extreme actual 
distance travelled was at 
least *1 of an inch. The 
movements of the cotyle- 
dons of two seedlings were 
observed; one facing a north- 
east window, and the other 
so feebly illuminated from 
above as to be almost in 
darkness. They continued 
to sink till about noon, 

when they began to rise ; but between 5 and 7 or 8 p.m. 
they either sank a little, or moved laterally, and then again 
began to rise. At 7 a.m. on the following morning those on 
the plant before the north-east window had opened so little 
that they stood at an angle of 73° above the horizon, and were 
not observed any longer. Those on the seedling which had 
been kept in almost complete darkness, sank during the whole 
day, without rising about mid-clay, but rose during the night. 
On the third and fourth days they continued sinking without 
any alternate ascending movement; and this, no doubt, was 
due to the absence of light. 

Primula Sinensis (Primulacere). — A seedling was placed with 
the two cotyledons parallel to a north-east window on a day 
when the light was nearly uniform, and a filament was affixed 
to one of them. From observations subsequently made on 
another seedling with the stem secured to a stick, the greater 
part of the movement shown in the annexed figure (Fig. 33), 
must have been that of the hypocotyl, though the cotyledons 
certainly move up and clown to a certain extent both during the 
day and night. The movements of the same seedling were traced 



HelianfJius annuus : eircumnutation of 
hypocotyl, with filament fixed across 
its summit, traced on a horizontal 
glass in darkness, from 8.45 A.M. to 
10.45 p.m., and for an hour on follow- 
ing morning. Movement of bead 
magnified 21 times, here reduced to 
one-half of original scale. 



m 



CIRCUMNUTATION OF SEEDLINGS. Chap. 1 



on the following day with, nearly the same result; and there 
can be no doubt about the circumnutation of the hypocotyl. 



Fig. 33. 




Primula Sinensis : conjoint circumnutation of hypocotyl and cotyledon, 
traced on vertical glass, from 8.40 A.M. to 10.45 P.M. Movements of 
bead magnified about 26 times. 

Cyclamen Persicum (Primulaceao). — This plant is generally sup- 
posed to produce only a single cotyledon, but Dr. H. G-ressner * 
has shown that a second one is developed after a long interval 
of time. The hypocotyl is converted into a globular corm, even 
before the first cotyledon has broken through the ground with its 
blade closely enfolded and with its petiole in the form of an arch, 
like the arched hjpocotyl or epicotyl of any ordinary dicotyle- 
donous plant. A glass filament was affixed to a cotyledon, '55 
of an inch in height, the petiole of which had straightened itself 
and stood nearly vertical, but with the blade not as yet fully 
expanded. Its movements were traced during 24 2 h. on a 

horizontal glass, magnified 50 

\ Z 



34. 



times ; and in this interval it 
described two irregular small 
circles; it therefore circum nu- 
tates, though on an extremely 
small scale. 

Stapelia mrpedon (Ascle- 
piadese). — This plant, when 
mature, resembles a cactus. 
The flattened hypocotyl is 
fleshy, enlarged in the upper 
part, and bears two rudimen- 
tary cotyledons. It breaks 
through the ground in an arched form, with the rudimentary 
cotyledons closed or in contact. A filament was affixed almost 



\H "Vr^; 



Stapelia sarpedon : circumnutation 
of hypocotyl, illuminated from 
above, traced on horizontal glass, 
from 6.45 A.M. June 26th to 8.45 
a.m. 28th. Temp. 23°-24° C. 
Movement of bead magnified 21 
times 



* 'Bot. Zeitung,' 1S74, p. 837. 



Ohai. I. IPOMCEA. 47 

vertically to the hypocotyl of a seedling half an inch high ; and 
its movements were traced during 50 h. on a horizontal glass 
(Fig. 34). From some unknown cause it bowed itself to one 
side, anl as this was effected by a zigzag course, it probably 
circumnutated ; but with hardly any other seedliDg observed 
by us was this movement so obscurely shown. 

' Ipomoea ccerulea vel Pharbitis nil (Convolvulacese). — Seedlings 
of this plant were observed because it is a twiner, the upper 
internodes of which circumnutate conspicuously; but, like 
other twining plants, the first few internodes which rise above 
the ground are stiff enough to support themselves, and therefore 
do not circumnutate in any plainly recognisable maimer.^ In 
this particular instance the fifth internode (including the hypo- 
cotyl) was the first which plainly circumnutated and twined 
round a stick. We therefore wished to learn whether circum- 
nivtation could be observed in the hypocotyl if carefully observed 
in our usual manner. Two seedlings were kept in the dark 
with filaments fixed to the upper part of their hypocotyls ; but 
from circumstances not worth explaining their movements were 
traced for only a short time. One moved thrice forwards and 
twice backwards in nearly opposite directions, in the course of 
3 h. 15 m. ; and the other twice forwards and twice backwards 
in 2 h. 22 m. The hypocotyl therefore circumnutated at a re- 
markably rapid rate. It may here be added that a filament was 
affixed transversely to the summit of the second internode above 
the cotyledons of a little plant 3? inches in height; and its 
movements were traced on a horizontal glass. It circumnutated, 
and the actual distance travelled from side to side was a quarter 
of an inch, which was too. small an amount to be perceived with- 
out the aid of marks. 

The movements of the cotyledons are interesting from their 
complexity and rapidity, and in some other respects. The 
hypocotyl (2 inches high) of a vigorous seedling was secured to a 
stick, and a filament with triangles of paper was affixed to one 
of the cotyledons. The plant was kept all day in the hot-house, 
and at 4.20 p.m. (June 20th) was placed under a skylight in 
the house, and observed occasionally during the evening and 
night. It fell in a slightly zigzag line to a moderate extent 
from 4.20 p.m. till 10 15 p.m. When looked at shortly after mid- 
night (12.30 p.m.) it had risen a very little, and considerably bj 



Movements and Habits of Climbing Plants/ p. 33, 1875. 



48 



CISCUMNUTATION OF SEEDLINGS. Chap. I 



3.45 am. When again looked 
Fig. 35'. 



\5°j>.m. 




tf'.io'.ujn 21 s ? 



IO~30.jp.Ta.21 

[pomcea ccsrulea : circumnutation of 
cotyledon, tracod on vertical glass, 
from 6.10 a.m. June 21st to 6.45 
a.m. 22nd. Cotyledon with petiole 
1'6 inch in length, apex of blade 
4*1 inch from the vertical glass; 
so movement not greatly mag- 
nified ; temp. 20° C. 



at, at 6.10 a.m. (21st), it had 
fallen largely. A new tracing 
was now begun (see Fig. 35), 
and soon afterwards, at 6.42 
a.m., the cotyledon had risen a 
little. During the forenoon it 
was observed about every 
hour; but between 12.30 and 
6 p.m. every half-hour. If the 
observations had been made at 
these short intervals during the 
whole day, the figure would 
have been too intricate to have 
been copied. As it was, the 
cotyledon moved up and down 
in the course of 16 h. 20 m. (i e. 
between 6.10 a.m. aud 10.30 
p.m.) thirteen times. 

The cotyledons of this seed- 
ling sank downwards during 
both evenings and the early 
part of the night, but rose 
during the latter part. As this 
is an unusual movement, the 
cotyledons of twelve other seed- 
lings were observed ; they stood 
almost or quite horizontally at 
mid-day, and at 10 p.m. were 
all declined at various angles. 
The most usual angle was be- 
tween 30° and 35°; but three 
stood at about 50° and one at 
even 70° beneath the horizon. 
The blades of all these cotyle- 
dons had attained almost their 
full size, viz. from 1 to li inches 
in length, measured along their 
midribs. It is a remarkable 
fact that whilst young — that 
is, when less than half an inch 
in length, measured in tire 
same manner— they do not sink 



Chap. T. CERINTHE. 49 

downwards in the evening. Therefore their weight, which is 
considerable when almost fully developed, probably came into 
play in originally determining the downward movement. The 
periodicity of this movement is much influenced by the degree 
of light to which the seedlings have been exposed during the 
day; for three kept in an obscure place began to sink about 
noon, instead of late in the evening ; and those of another seed- 
ling were almost paralysed by having been similarly kept during 
two whole days. The cotyledons of several other species if 
Ipomcea likewise sink downwards late in the evening. 

Cerinthe major (Boragineae). — The circumnutation of the 
hypocotyl of a young seedling with the cotyledons hardly 

Fig. 36. 




Cerinthe major: circumnutation of hypocotyl, with filament fixed across its 
summit, illuminated from above, traced on horizontal glass, from 
9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magnified. 
30 times, here reduced to one-third of original scale. 

expanded, is shown in the annexed figure (Fig. 36), which 
apparently represents four or five irregular ellipses, described 
in the course of a little over 12 hours. Two older seedlings 
were similarly observed, excepting that one of them was kept 
in the dark; their hypocotyls also circumnutated, but in a more 
simple manner. The cotyledons on a seedling exposed to the 
light fell from the early morning until a little after noon, and 
then continued to rise until 10.30 p.m. or later. The cotyledons 
of this same seedling acted i n the same general manner during 
the two following days. It had previously been tried in the 
dark, and after being thus kept for only 1 h. 40 m. the cotyledons 
began at 4.30 p.m. to sink, instead of continuing to rise till late 
at night. 



w> 



CIRCUMXUTATIOX OF SEEDLINGS. 



Chap. I 



Fig. 37. 



NoJana prostrata (NolanesB). — The movements were not 
traced, but a pot with seedlings, which had been kept in the 
dark for an hour, was placed under the microscope, with the 
micrometer eye-piece so adjusted that each division equalled 
g^oth of an inch. The apex of one of the cotyledons crossed 
rather obliquely four divisions in 13 minutes ; it was also sink- 
ing, as shown by getting out of focus. The seedlings were 
again placed in darkness for another hour, and the apex now 
crossed two divisions in 6 m. 18 s. ; that is, at very nearly the 
same rate as before. After another interval of an hour in dark- 
ness, it crossed two divisions in 4 m. 15 s., there- 
fore at a quicker rate. In the afternoon, after a 
longer interval in the dark, the apex was motion- 
less, but after a time it recommenced moving, 
though slowly ; perhaps the room was too cold, 
Judging from previous cases, there can hardly 
be a doubt that this seedling was circumnu ra- 
ting. 

Solarium ly copers icum (Solaneae) — The move- 
ments of the hypocotyls of two seedling to- 
matoes were observed during seven hours, and 
there could be no doubt that both circumnu- 
tated. They were illuminated from above, but 
by an accident a little light entered on one side, 
across its sum- and in the accompanying figure (Fig. 37) it 

mit, traced on 
horizontal glass, 
from 10 a.m. to 
5 p.m. Oct. 24th. 
Illuminated ob- 
liquely from 
above. Move- 
ment of bead 



Solanum lycoper- 
sicum : circum- 
nu tation of hy- 
pocotyl, with 
filament fixed 



may be seen that the hypocotyl moved to this 
side (the upper one in the figure), making small 
loops and zigzagging in its course. The move- 
ments of the cotyledons were also traced both 
on vertical and horizontal glasses ; their angles 
with the horizon were likewise measured at 
magnified about various hours. They fell from 8.30 a.m. (October 
35 times, here 17th) to about noon ; then moved laterally in a 
reduced to one- z jp. za g u ne anc [ a t about 4 p.m. began to rise ; 
third of original & & ' ° 

scale- they continued to do so until 10.30 p.m., by 

which hour they stood vertically and were asleep. 
At what hour of the night or early morning they began to fall 
was not ascertained. Owing to the lateral movement shortly 
after mid-day, the descending and ascending lines did not 
coincide, and irregular ellipses were described during each 24 h. 
The regular periodicity of these movements is destroyed, as we 
shall hereafter see, if the seedlings are kept in the dark. 



Uhap. l solanum. 61 

Solanum palinacanthum. — Several arched hypocotyls rising 
nearly *2 of an inch above the ground, but with the cotyledons 
still buried beneath the surface, were observed, and the tracings 
showed that they circumnutated. Moreover, in several cases 
little open circular spaces or cracks in the argillaceous sand 
which surrounded the arched hypocotyls were visible, and 
these appeared to have been made by the hypocotyls having 
bent first to one and then to another side whilst growing up- 
wards. In two instances the vertical arches were observed to 
move to a considerable distance backwards from the point where 
the cotyledons lay buried; this movement, which has been 
noticed in some other cases, and which seems to aid in extracting 
the cotyledons from the buried seed-coats, is due to the com- 
mencement of the straightening of the hypocotyl. In order to 
prevent this latter movement, the two legs of an arch, the 

Fig. 38. 




Solanum palinacanthum: eircumnutatiou of an arched hypocotyl, just 
emerging from the ground, with the two legs tied together, traced in 
darkness on a horizontal glass, from 9.20 A.M. Dec. 17th to 8.30 A.M. 
19th. Movement of bead magnified 13 times; but the filament, which 
was affixed obliquely to the crown of the arch, was of unusual length. 

summit of which was on a level with the surface of the soil, 
were tied together ; the earth having been previously removed 
to a little depth all round. The movement of the arch during 
47 hours under these unnatural circumstances is exhibited 
in the annexed figure. 

The cotyledons of some seedlings in the hot-house were hori- 
zontal about noon on December 13th ; and at 10 p.m. had risen 
to an angle of 27° above the horizon ; at 7 a.m. on the following 



52 



CTRCUMNUTATION OF SEEDLINGS. Ciiak 1 



Fig. 39. 




morning, before it was light, they had risen to 59° above the 
horizon; in the afternoon of the same day they were found 
again horizontal. 

Beta vulgaris (Chenopodese). — The seedlings are excessively 
sensitive to light, so that although on the first day they 
were uncovered only during two or three 
minutes at each observation, they all moved 
steadily towards the side of the room 
whence the light proceeded, and the trac- 
ings consisted only of slightly zigzag lines 
directed towards the light. On the next 
day the plants were placed in a completely 
darkened room, and at each observation 
were illuminated as much as possible from 
vertically above by a small wax taper. The 
annexed figure (Fig. 39) shows the move- 
ment of the hypocotyl during 9 h. under 
these circumstances. A second seedhng 
was similarly observed at the same time, 
and the tracing had the same peculiar 
character, due to the hypocotyl often mov- 
ing and returning in nearly parallel lines. 
The movement of a third hypocotyl differed 
greatly. 

We endeavoured to trace the movements 
of the cotyledons, and for this purpose 
some seedlings were kept in the dark, but 
they moved in an abnormal manner ; they 
continued rising from 8.45 a.m.- to 2 p.m., 
then moved laterally, and from 3 to 6 p.m. 
descended ; whereas cotyledons which have been exposed all 
the day to the light rise in the evening so as to stand verti- 
cally at night; but this statement applies only to young 
seedlings. For instance, six seedlings in the greenhouse had 
their cotyledons partially open for the first time on the morning 
of November 15th, and at 8.45 p.m. all were completely closed, 
so that they might properly be said to be asleep. Again, on the 
morning of November 27th, the cotyledons of four other seedlings, 
which were surrounded by a collar of brown paper so that they 
received light only from above, were open to the extent of 
39°; at 10 p.m. they were completely closed; next morning 
(November 28th) at 6.45 a,m., whilst ft was still dark, two of them 



Betatu'garis: circum- 
nutatioa of hypo- 
cotyl, with filament 
fixed obliquely a- 
cross its summit, 
traced in darkness 
on horizontal glass, 
from 8.25 a.m. to 
5.30 P.M. Nov. 4th. 
Movement of bead 
magnified 23 times, 
here reduced to one- 
third of original 
scale. 



Ohap. I. RICINUS AND QUEECUS. 53 

were partially open and all opened in the course of the morning ; 
but at 10.20 p.m. all four (not to mention nine others which 
had been open in the morning and six others on another occa- 
sion) w r ere again completely closed. On the morning of the 
29th they were open, but at night only one of the four w r as 
closed, and this only partially; the three others had their 
cotyledons much more raised than during the day. On the 
night of the 30th the cotyledons of the four were only slightly 
raised. 

Ricinus Borhouiensis (Euphorbiaceae).— Seeds were purchased 
under the above name — probably a variety of the common castor- 
oil plant. As soon as an arched hypocotyl had risen clear above 
the ground, a filament was attached to the upper leg bearing the 
cotyledons which were still buried beneath the surface, and the 
movement of the bead was traced on a horizontal glass during 
a period of 34 h. The lines traced were strongly zigzag, and 
as the bead twice returned nearly parallel to its former course 
in two different directions, there could be no doubt that the 
arched hypocotyl circumnutated. At the close of the 34 h. 
the upper part began to rise and straighten itself, dragging the 
cotyledons out of the ground, so that the movements of the 
bead could no longer be traced on the glass. 

Quercus (American sp.) (Cupuliferas). — Acorns of an American 
oak which had germinated at Kew were planted in a pot in 
the greenhouse. This transplantation checked their growth; 
but after a time one grew to a height of five inches, 
measured to the tips of the small partially unfolded leaves on 
the summit, and now looked vigorous. It consisted of six 
very thin internodes of unequal lengths. Considering these 
circumstances and the nature of the plant, we hardly expected 
that it would circumnutate ; but the annexed figure (Fig. 40) 
shows that it did so m a conspicuous manner, changing its 
course many times and travelling in all directions during the 
43 h. of observation. The figure seems to represent 5 or 6 
irregular ovals or ellipses. The actual amount of movement 
from side to side (excluding one great bend to the left) was 
about *2 of an inch; but this was difficult to estimate, as owing 
to the rapid growth of the stem, the attached filament was 
much further from the mark beneath at the close than at the 
commencement of the observations. It deserves notice that the 
pot was placed in a north-east room within a deep box, the top 
of which was not at first covered up, so that the inside facing 



54 



ClliCUMNUTATION OF SEEDLINGS. Chap. L 



the windows was a little more illuminated than the opposite 
side; and during the first morning the stem travelled to a 
greater distance in this direction (to the left in the figure) than 
it did afterwards when the box was completely protected from 
light 

Fig. 40. 




Qwrcus (American sp.) : circumnutation of young stem, traced on hori- 
zontal glass, from 12.50 P.M. Feb. 22nd to 12.50 P.M. 24th. -Movement 
of bead gieatly magnified at first, but slightly towards the close of the 
observations — about 10 times on an average. 

Quercus robur. — Observations were made only on the move- 
ments of the radicles from germinating acorns, which were allowed 
to grow downwards in the manner previously described, over 
plates of smoked glass, inclined at angles between 65° and 69° 
to the horizon. In four cases the tracks left were almost straight, 
but the tips had pressed sometimes with more and sometimes 
with less force on the glass, as shown by the varying thickness 
of the tracks and by little bridges of soot left across them. 
In the fifth case the track was slightly serpentine, that is, the 
tip had moved a little from side to side. In the sixth case 
(Fig. 41, A) it was plainly serpentine, and the tip had pressed 
almost equably on the glass in its whole course. In the seventh 
case (B) the tip had moved both laterally and had 



Chap. L QUEECUS AND COEYLUS. 55 

alternately with -unequal force on the glass; so that it had 
moved a little in two planes at right angles to one another. In 
the eighth and last case (C) it had moved very little laterally, 
but had alternately left the glass and come into contact with it 
again. There can be no doubt that in the last four cases the 
ladicle of the oak circumnutated whilst growing downwards. 

Fig. 41. 




\ 



Quercus rohur : tracks left on inclined smoked glass-plates by tips of 
radicles in growing downwards. Plates A and C inclined at 65° and 
plate B at 68° to the horizon. 

Cdrylus aveJIana (Cory lacese).— The epicotyl breaks through 
the ground in an arched form ; but in the specimen which was 
first examined, the apex had become decayed, and the epicotyl 
grew to some distance through the soil, in a tortuous, almost 
horizontal direction, like a root. In consequence of this injury 
it had emitted near the hypogean cotyledons two secondary 
shoots, and it was remarkable that both of these were arched, 
like the normal epicotyl in ordinary cases. The soil was removed 
from around one of these arched secondary shoots, and a glass 
filament was affixed to the basal leg. The whole was kept 
damp beneath a metal-box with a glass lid, and was thus illumi- 
nated only from above. Owing apparently to the lateral pressure 
of the earth being removed, the terminal and bowed-down part 
of the shoot began at once to move upwards, so that after 
24 h. it formed a right angle with the lower part. This lower 
part, to which the filament was attached, also straightened 
itself, and moved a little backwards from the upper part. Con- 
sequently a long line was traced on the horizontal glass ; and 
5 



Ml 



CIKCUMNUTATION OF SEEDLINGS. 



Chap. 1 



Fisc. 42. 



this was in parts straight and in parts decidedly zigzag, 

indicating circnmnutation. 
On the following day the other secondary shoot was observed ; 

it was a little more advanced in age, for the upper part, instead 
of depending vertically downwards, 
stood at an angle of 45° above the 
horizon. The tip of the shoot pro- 
jected obliquely "4 of an inch above 
the ground,- but by the close of our 
observations, which lasted 47 h., it 
had grown, chiefly towards its base., 
to a height of ■ 85 of an inch. The 
filament was fixed transversely to 
the basal and almost upright half 
of the shoot, close beneath the lowest 
scale-like appendage. The circum- 
nutating course pursued is shown 
in the accompanying figure (Fig. 
42). The actual distance traversed 
from side to side was about '04 of 
an inch. 

Finns pinaster (Conifers). — A 
young hypocotyl, with the tips 
of the cotyledons still enclosed 
within the seed-coats, was at -first 

only -35 of an inch in height; but the upper part grew so 

rapidly that at the end of our observations it was ■ 6 in height, 




\ 

Corylus avellana: circumnuta- 
tion of a young shoot emitted 
from the epicotyl, the apex 
of which had been injured, 
traced on a horizontal glass, 
from 9 A.M. Feb. 2nd to 8 
a.m. 4th. Movement ol 
bead magnified about 27 
times. 



Fig. 43 



**\ 




Pinus pinaster : circumnutation of hypocotyl, with filament fixed across its 
summit, traced on horizontal glass, from 10 A.M. March 21st to 9 A.M. 
23rd. Seedling kept in darkness. Movement of bead magnified abcu* 
35 times. 



Chaf. I. PINUS AND CYCAS. 57 

and by this time the filament was attached some way down the 
little stem. From some unknown cause, the hypocotyl moved 
far towards the left, but there could be no doubt (Fig. 43) that 
it circumnutated. Another hypocotyl was similarly observed, 
and it likewise moved in a strongly zigzag line to the same side. 
This lateral movement was not caused by the attachment of 
the glass filaments, nor by the action of light ; for no light was 
allowed to enter when each observation was made, except from 
vertically above. 

The hypocotyl of a seedling was secured to a little stick ; it 
bore nine in appearance distinct cotyledons, arranged in a circle. 
The movements of two nearly opposite ones were observed. The 
tip of one was painted white, with a mark placed below, and the 
figure described (Fig. 44, A) shows that it made an irregular 

Ficr. 44. 





A. 

Pinus pinaster : circumnutation of two opposite cotyledons, traced on 
horizontal glass in darkness, from 8.45 A.M. to 8.35 P.M. Nov. 25th. 
Movement of tip in A magnified about 22 times, here reduced to one- 
half of original scale. 

circle in the course of about 8 h. During the night it 
travelled to a considerable distance in the direction indicated 
by the broken line. A glass filament was attached longitu- 
dinally to the other cotyledon, and this nearly completed 
(Fig. 44, B) an irregular circular figure in about 12 hours. 
During the night it also moved to a considerable distance, in 
the direction indicated by the broken line. The cotyledons 
therefore circumnutate independently of the movement of the 
hypocotyl. Although they moved much during the night, they 
did not approach each other so as to stand more vertically than 
dui-ing the day. 



58 CIRCUMNUTATION OF SEEDLINGS. Chap I 

Cycas-vectinata (Cycadeae). — The large seeds of this plant in 
germinating first protrude a single leaf, which breaks through 
the ground with the petiole bowed into an arch and with the 
leaflets involuted. A leaf in this condition, which at the close 
of our observations was 2i inches in height, had its movements 
traced in a warm greenhouse by means of a glass filament 
bearing paper triangles attached across its tip. The tracing 
(Fig. 45) slows how large, complex, and rapid were the circ urn- 
Fig. 45. 




Cycas pectinata : circumnutation of young leaf whilst emerging from the 
ground, feebly illuminated from above, traced on vertical glass, from 
5 P.M. May 28th to 11 A.M. 31st. Movement magnified 7 times, here 
reduced to two-thirds of original scale. 

nutating movements. The extreme distance from side to side 
which it passed over amounted to between *6 and *7 of an 
inch. 

fauna Warscewiczii (Cannacese).— A seedling with the plu- 
mule projecting one inch above the ground was observed, but 
not under fair conditions, as it was brought out of the hot- 
house and kept in a room not sufficiently warm. Nevertheless 
the tracing (Fig. 46) shows that it made two or three incom- 
plete irregular circles or ellipses in the course of 48 hours. The 
plumule is straight ; and this was the first instance observed 



Cuap. L 



ALLIUM. 



59 



by us of the part that first breaks through the ground not 
being arched. 

Fig, 46. 




Canna Warscevciczii : circumnutation of plumule with filament affixed 
obliquely to outer sheath-like leaf, traced in darkness onhorizontal glass 
from 8.45 a.m. Nov. 9th to 8.10 a.m. 11th. Movement of bead mag- 
nified 6 times. 

Allium cepa (Liliaceae).— -The narrow green leaf, which pro- 
trudes from the seed of the common onion as a cotyledon,* 
breaks through the grouud in the form of an arch, in the same 
manner as the hypocotyl or epicotyl of a dicotyledonous plant. 
Long after the arch has risen above the surface the apex 
remains within the seed-coats, evidently absorbing the still 
abundant contents. The summit or crown of the arch, when 
it first protrudes from the seed and is still buried beneath the 
ground, is simply rounded; but before it reaches the surface 
it is developed into a conical protuberance of a white colour 
(owing to the absence of chlorophyll), whilst the adjoining parts 
are green), with the epidermis apparently rather thicker and 
tougher than elsewhere. We may therefore conclude that this 
conical protuberance is a special adaptation for breaking through 
the ground,f and answers the same end as the knife-like white 
crest on the summit of the straight cotyledon of the Graminese. 



* This is the expression used 
by Sachs in his ' Text-book of 
Botany.' 

t Haberlandt has briefly de- 
scribed ('Die Schutzeinrichtun- 
gen . . . Keimpflanze,' 1877, p. 77) 
this curious structure and the 



pitrpose which it subserves. He 
states that good figures of the 
cotyledon of the onion have been 
given by Tittmann and by Sachs 
in his ' Experimental Physiologie,' 
p. 93. 



60 



CIKCUMNUTATION OF SEEDLINGS. 



Chap. I. 



Fig. 47. 



After a time the apex is drawn out of the empty seed-coats, 
and rises up, forming a right angle, or more commonly a still 
larger angle with the lower part, and occasionally the whole 
becomes nearly straight. The conical protuberance, which 
originally formed the crown of the arch, is now seated on one 
side, and appears like a joint or knee, which from" acquiring 
chlorophyll becomes green, and increases in size. In rarely ox 
never becoming perfectly straight, these cotyledons differ remark- 
ably from the ultimate condition of the arched hypocotyls or 
epicotyls of dicotyledons. It is, also, a singular circumstance 
that the attenuated extremity of the upper bent portion 
invariably withers and dies. 

A filament, 1*7 inch in length, was affixed nearly upright 
beneath the knee to the basal and vertical portion of a 
cotyledon; and its movements were 
traced during 14 h. in the usual manner. 
The tracing here given (Fig. 47) indi- 
cates circumnutation. The movement of 
the upper part above the knee of the same 
cotyledon, which projected at about an 
angle of 45° above the horizon, was 
observed at the same time. A filament 
was not affixed to it, but a mark was 
placed beneath the apex, which was 
almost white from beginning to wither, 
and its movements were thus traced. The 
figure described resembled pretty closely 
that above given ; and this shows that the 
chief seat of movement is in the lower or 
basal part of the cotyledon. 

Asparagus officinalis (Asparagese). — ■ 
The tip of a straight plumule or cotyledon 
(for we do not know which it should be 
called) was found at a depth of "1 inch 
beneath the surface, and the earth was then removed all round 
to the depth of "3 inch. A glass filament was affixed obliquely to 
it, and the movement of the bead, magnified 17 times, was traced 
in darkness. During the first lb. 15m. the plumule moved to 
the right, and daring the next two hours it returned in a roughly 
parallel but strongly zigzag course. From some unknown cause 
it had grown up through the soil in an inclined direction, and 
now through apogeotropism it moved during nearly '24 h. in 




Allium cepa : circumnu- 
tation of basal half, 
of arched cotyledon, 
traced in darkness on 
horizontal glass, from 
8.15 A.M. to 10 P.M. 
Oct. 31st. Movement 
of bead magnified 
about 17 times. 



Chap. 1 



ASPABAGU3. 



61 



tlie same general direction, but in a slightly zigzag manner, 
until it became upright. On the following morning it changed 
its course completely. There can therefore hardly be a doubt 
that the plumule circumnutates, whilst buried beneath the 
ground, as much as the pressure of the surrounding earth will 
permit. The surface of the soil in the pot was now covered with 
a thin layer of very fine argillaceous sand, which was kept damp; 
and after the tapering seedlings had grown a few tenths of 
an inch in height, each was found surrounded by a little open 
space or circular crack ; and this could be accounted for only by 
their having circumnutated and thus pushed away the sand on 
all sides ; for there was no vestige of a crack in any other part. 
In order to prove that there was circumnutation, the move- 

Ficr. 48. 





Asparagus officinalis : circumnutation of plumules with tips whitened and 
marks placed beneath, traced ou a Horizontal glass. A, young plumule ; 
movement traced from 8.30 a.m. Nov. 30th to 7.15 A.M. next morning ; 
magnified about 35 times. B, older plumule ; movement traced from 
10.15 A.M. to 8.10 p.m. Nov. 29th ; magnified 9 times, but here reduced 
to one-half of original scale. 

ments of five seedlings, varying in height from '3 inch to 2 inches, 
were traced. They were placed within a box and illuminated 
from above; but in all five cases the longer axes of the figures 
described were directed to nearly the same point ; so that more 
light seemed to have come through the glass roof of the green- 
house on one side than on any other. All five tracings re- 
sembled each other to a certain extent, and it will suffice to give 
two of them. In A (Fig. 48) the seedling was only 45 of an 



62 CIRCUMNUTATION" OF SEEDLINGS. Chap. I 

inch in height, and consisted of a single internode bearing a 
bud on its summit. ' The apex described between 8.30 a..m. and 
10.20 p.m. (i.e. during nearly 14 hours) a figure which would 
probably have consisted of 3^ ellipses, had not the stem been 
drawn to one side until 1 p.m., after which hour it moved back- 
wards. Ou the following morning it was not far distant from 
the point whence it had first started. The actual amount of 
movement of the apex from side to side was very small, viz. 
about -jL-th of an inch. The seedling of which the movements 
are shown in Fig. 48, B, was If inch in height, and consisted of 
three internodes besides the bud on the summit. The figure, 
which was described during 10 h., apparently represents two 
irregular and unequal ellipses or circles. The actual amount of 
movement of the apex, in the line not influenced by the light, was 
•11 of an inch, and in that thus influenced ■ 37 of an inch. With 
a seedling 2 inches in height it was obvious, even without the 
aid of any tracing, that the uppermost part of the stem bent 
successively to all points of the compass, like the stem of a 
twining plant. A little increase in the power of circumnutating 
and in the flexibility of the stem, would convert the common 
asparagus into a twining plant, as has occurred with one species 
in this genus, namely, A. scandens. 

Phalaris Canarie.nsis (Gramineae). — "With the Graminese the 
part which first rises above the ground has been called by some 
authors the pileole; and various views have been expressed on 
its homological nature. It is considered by some great authori- 
ties to be a cotyledon, which term we will use without venturing 
to express any opinion on the subject.* It consists in the 
present case of a slightly flattened reddish sheath, terminating 
upwards in a sharp white edge; it encloses a true green leaf, 
which protrudes from the sheath through a slit-like orifice, 
close beneath and at right angles to the sharp edge on the 
summit. The sheath is not arched when it breaks through the 
ground. 

The movements of three rather old seedlings, about li inch 
in height, shortly before the protrusion of the leaves, were first 
traced. They were illuminated exclusively from above; for, ns 
will hereafter, be shown, they are excessively sensitive to thd 



* We are indebted to the Rev. this subject, together with r& 
G. H^nslow for an abstract of the ferences. 
dews which have been held on 



CUAP. I, 



PHALAEIS. 



08 



Fig. 49 




Phalaris Conan'ensis : circumnu- 
tation of a cotyledon, with a 
mark placed below the apex, 
traced on a horizontal glass, 
from 8.35 A.M. Nov. 26th to 
8.45 a.m. 27th. Movement of 
apex magnified 7 times, here 
reduced to one-half scale. 



action of light ; and if any enters even temporarily on one side, 

they merely bend to this side in slightly zigzag lines. Of the three 

tracings one alone (Fig. 49) is here given. Had the observations 

been more frequent during the 12 h. 

two oval figures would have been 

described with their longer axes at 

right angles to one another. The 

actual amount of movement of the 

apex from side to side was about 

•3 of an inch. The figures described 

by the other two seedlings resembled 

to a certain extent the one here 

given. 

A seedling which had just broken 
through the ground and projected 
only •Jgth of an inch above the 
surface, was next observed in the 
same manner as before. It was 
necessary to clear away the earth 
all round the seedling to a little 
depth in order to place a mark 

beneath the apex. The figure (Fig. 50) shows that the apex 
moved to one side, but changed its course ten times in the 
course of the ten hours of observa- 
tion ; so that there can be no doubt 
about its circumnutation. The 
cause of the general movement 
in one direction could hardly be 
attributed to the entrance of 
lateral light, as this was carefully 
guarded against ; and we suppose 
it was in some manner connected 
with the removal of the earth 
round the little seedling. 

Lastly, the soil in the same pot 
was searched with the aid of a 
lens, and the white knife-like apex 
of a seedling was found on an exact 
level with that of the surrounding 

surface. The soil was removed all round the apex to the depth 
of a quarter of an inch, the seed itself remaining covered. The 
pot, protected from lateral light, was placed under the micro- 




Phalaris Canariensis : circumnu- 
tation of a very young coty- 
ledon, with a mark placed 
below the apex, traced on a 
horizontal glass, from 11.37 
a.m. to 9.30 p.m. Dec. 13th. 
Movement of apex greatly 
magnified, here reduced to 
one-fov.rth of original scale. 



64 



CIRCUMNUTATION OF SEEDLINGS. 



Chap. L 



scope with a micrometer eye-piece, so arranged that each 
division equalled -^oth of an inch. After an interval of 30 in. 
the apex was observed, and it was seen to cross a little obliquely 
two divisions of the micrometer in 9 m. 15 s. ; and after a few 
minutes it crossed the same space in 8 m. 50 s. The seedling 
was again observed after an intervalof three-quarters of an hour, 
and now the apex crossed rather obliquely two divisions in 10 m. 
We may therefore conclude that it was travelling at about the 
rate of ^tli of an inch in 45 minutes. We may also conclude 
from these and the previous observations, that the seedlings of 
Phalaris in breaking through the surface of the soil circum- 
nutate as much as the surrounding pressure will permit. This 
fact accounts (as in the case before given of the asparagus) for 
a circular, narrow, open space or crack being distinctly visible 
round several seedlings which had risen through very fine 
argillaceous sand, kept uniformly damp. 

Zea mays (Grarninepe). — A glass filament was fixed obliquely 

to the summit of a cotyledon, 
Fig. 51. rising - 2 of an inch above the 

ground ; but by the third morn- 
ing it had grown to exactly 
thrice this height, so that the 
distance of the bead from the 
mark below was greatly in- 
creased, consequently the trac- 
ing (Fig. 51) was much more 
magnified on the first than on 
the second day. The upper 
part of the cotyledon changed 
its course by at least as much 
as a rectangle six times on each 
of the two days. The plant 
was illuminated by an obscure 
light from vertically above. 
This was a necessary precau- 
tion, as on the previous day we had traced the movements of 
cotyledons placed in a deep box, the inner side of which was 
feebly illuminated on one side from a distant north-east window, 
and at each observation by a wax taper held for a minute or 
two on the same side ; and the result was that the cotyledons 
travelled all day long to this side, though making in their course 
some conspicuous flexures, from which fact alone we might have 




Zea mays : circumnutation of cotyle- 
don, traced on horizontal glass, from 
8.30 A.M. Feb. 4th to 8 a.h. 6th. 
Movement of bead magnified on an 
average about 25 times. 



Chap. I. PHALAKIS 65 

concluded that they were circumnutating ; but we thought it 
advisable to make the tracing above given. 

Radicles. — Glass filaments were fixed to two short radicles, 
placed so as to stand almost upright, and whilst bending down- 
wards through geotropism their courses were strongly zigzag ; 
from this latter circumstance circumnutation might have been 
inferred, had not their tips become slightly withered after the 
first 24 h., though they were watered and the air kept very 
damp. Nine radicles were next arranged in the manner 
formerly described, so that in growing downwards they left 
tracks on smoked glass-plates, inclined at various angles between 
45° and 80° beneath the horizon. Almost every one of these 
tracks offered evidence in their greater or less breadth in dif- 
ferent parts, or in little bridges of soot being- 
left, that the apex had come alternately into Fig. 52. 
more and less close contact with the glass. In 
the accompanying figure (Fig. 52) we have 
an accurate copy of one such track. In two 
instances alone (and in these the plates were 
highly inclined) there was some evidence of 
slight lateral movement. We presume therefore 
that the friction of the apex on the smoked 
surface, little as this could have been, sufficed | 

to check the movement from side to side of these ,-, L , 

, .. , ... , Zea mays : track 

delicate radicles. left on incliued 

Avena saliva (Gramineze). — A cotyledon, lh smoked glass- 
inch in height, was placed in front -of a north- P late b ^ t |P 
east window, and the movement of the apex growLgdown- 
was traced on a horizontal glass during two wards, 
days. It moved towards the light in a slightly 
zigzag line from 9 to 11.30 a.m. on October 15th ; it then moved 
a little backwards and zigzagged much until 5 p.m., after which 
hour, and during the night, it continued to move towards the 
window. On the following morning the same movement was 
continued in a nearly straight line until 12.40 p.m., when the sky 
remained until 2.35 extraordinarily dark from thunder-clouds. 
During this interval of 1 h. 55 m., whilst the light was obscure, 
it was interesting to observe how circumnutation overcame 
heliotropism, for the apex, instead of continuing to move towards 
the window in a slightly zigzag line, reversed its course four 
times, making two small narrow ellipses. A diagram of this case 
will be given in the chapter on Heliotropism. 



66 



CIKCUMNUTATION OF SEEDLINGS. Chap, t 



A filament was next fixed to a cotyledon only £ of an inch in 
height, which was illuminated exclusively from above, and as 
it was kept in a warm greenhouse, it grew rapidly ; and now 
there could be no doubt about its circumnutation, for it described 
a figure of 8 as well as two small ellipses in 5j hours. 

Nephrodium molle (Filices). — A seedling fern of this species 



*'ig. 53. 




came up by chance in a flower- 
pot near its parent. The frond, 
as yet only slightly lobed, was 
only '16 of an inch in length and 
•2 in breadth, and was supported 
on a rachis as fine as a hair 
and *23 of an inch in height. A 
very thin glass filament, which 
projected for a length of '36 of 
an inch, was fixed to the end of 
the frond. The movement was 
so highly magnified that the 
figure (Fig. 53) cannot be fully 
trusted; but the frond was 
constantly moving in a complex 
manner, and the bead greatly 
changed its course eighteen times in the 12 hours of observation. 
Within half an hour it often returned in a line almost parallel 
to its former course. The greatest amount of movement occurred 
between 4 and 6 p.m. The circumnuta- 
tion of this plant is interesting, because 
the species in the genus Lygodium are 
well known to circumnutate conspicuously 
and to twine round any neighbouring 
object. 

Selaginella Kraussii (?) (Lycopodiaceae). 
— A very young plant, only '4 of an inch 
young plant, kept in j n height, had sprung up in a pot in the 
8*45*1*1*1? lVlm *ot-house. Al1 extremely fine glass fila- 
Oct. 31st. ment was fixed to the end of the frond- 

like stem, and the movement of the bead 
traced on a horizontal glass. It changed its course several 
times, as shown in Fig. 54, whilst observed during 13 h. 15 m., 
and returned at night to a point not far distant from that 
whence it had started in the morning. There can be no doubt 
ihat this little plant circumnutated 



Nephrodium molle : circumnutation 
of very young frond, traced in 
darkness on horizontal glass, 
from 9 A M. to 9. p.m. Oct. 30th. 
Movement of bead magnified 48 
times. 



Fig. 54. 



Selaginella Kraussii (?) : 
circumnutation of 



Chap. II. CIEOUMNUTATION OF SEEDLINGS. 67 



CHAPTER II. 

General Con^iperat'ons on the Movements and Growth o» 
Seedling Plants. 

Generality of the cireumimtating movement — Eadicles, their circum- 
nutation of service — Manner in which they penetrate the ground — : 
Manner in which hypocotyls and other organs break through the 
ground by being arched — Singular manner of germination in Megar. 
rhiza, &c. — Abortion of cotyledons— Circumnutation of hypocotyls 
and epicotyls whilst still buried and arched — Their power of 
straightening themselves — Bursting of the seed-co r tts— Inherited 
effect of the arching process in hypogean hypocotyls— Circumnuta- 
tion of hypocotyls and epicotyls when erect — Cii cumnutation of 
cotyledons — Pulvini or joints of cotyledons, duration of their 
activity, rudimentary in Oxalis corniculata, their development — 
Sensitiveness of cotyledons to light and consequent disturbance of 
their periodic movements — Sensitiveness of cotyledons to contact. 

The cireumimtating movements of the several parts 
or organs of a considerable number of seedling plants 
have been described in the last chapter. A list is here 
appended of the Families, Cohorts, Sub-classes, &c, 
to which they belong, arranged and numbered ac- 
cording to the classification adopted by Hooker.* 
Any one who will consider this list will see that the 
young plants selected for observation, fairly represent 
the whole vegetable series excepting the lowest 
cryptogams, and the movements of some of the lattei 
when mature will hereafter be described. As all the 
seedlings which were observed, including Conifers. 
Cycads and Ferns, which belong to the most ancient 



* As given in the ' General System of Botany.' by Le Maout and 
Decaisne, 1873. 



68 



CffiCUMXUTATIOX OF SEEDLINGS. Ce^p. H, 



types amongst plants, were continually circumnu- 
tating, we may infer that this kind of movement is 
common to every seedling species. 

Sub-Kingdom I. — Phaeiiogamous Plants. 
Class I.— Dicotyledons. 



Sub-class I 



Familv. 

14. Cruciferce. 

26. Caryophyllece. 

36 Malvacece. 

41. Oxalidece. 

49. Tropoeolece. 

52. Aurantiaccce. 

70. Hippocastanece. 

75. Leguminosce. 
106. Cucurbitacece. 
109. Cactece. 
122. Composites. 
135. Primulacece. 
145. Asclepiadece. 
151. Convolvulaccce. 
154. Borraginece. 

156. Isolanece. 

157. Solanece. 
181. Chenopodiece. 
202. Euphorbiaceoe. 

211. Cupuliferce. 

212. Corylaceas. 



—Angiosperms. 

Cohort. 
II. Parietales. 

IV. CARYOPHTLLALEi 

VI. Maxyales. 
VII. Geraniales. 
Ditto 
Ditto 
X. Sapindales. 

XI. ROSALES. 
XII. PASSIFLO RALES. 
XIV. FlCOIDALES. 
XVII. ASTRALES. 

XX. Primulales. 
XXII. Gentianales. 
xxiii. polemoxialks. 
Ditto 
Ditto 
XXIV. Solaxales. 
xxvii. chenopodiale& 
xxxii. euphorbiales. 
xxxvi. querxales 
Ditto 



Sub-class II. — Gymnosperms. 

223. Coniferce. 

224. Cycadeae. 

Class II.-— Monocotyledons. 



2. Cannaceoe, 
34. Liliacecc. 
41. Asparagccc. 
55. GramincGC. 



II. Amojiales. 

XI. LlLIALES. 

Ditto 
XV. Glumales. 



Sub-Kingi>om II. — Cryptcgamic Plants. 

1. Filices. I. FlLICALES. 

6. LycopodiacccB. Ditto 



Chap. II. ACTION OF THE RADICLE. 69 

Radicles. — In all the germinating seeds observed 
by us, the first change is the protrusion of the 
radicle, which immediately bends downwards and 
endeavours to penetrate the ground. In order to 
effect this, it is almost necessary that the seed should 
be pressed clown so as to offer some resistance, unless 
indeed the soil is extremely loose ; for otherwise the 
seed is lifted up, instead of the radicle penetrating 
the surface. But seeds often get covered by earth 
thrown up by burrowing quadrupeds or scratching 
birds, by the castings of earth-worms, by heaps of 
excrement, the decaying branches of trees, &c, and 
will thus be pressed clown ; and they must often fall 
into cracks when the ground is dry, or into holes. 
Even with seeds lying on the bare surface, the first 
developed root-hairs, by becoming attached to stones 
or other objects on the surface, are able to hold down 
the upper part of the radicle, whilst the tip pene- 
trates the ground. Sachs has shown* how well and 
closely root-hairs adapt themselves by growth to the 
most irregular particles in the soil, and become firmly 
attached to them. This attachment seems to be 
effected by the softening or liquefaction of the outer 
surface of the wall of the hair and its subsequent 
consolidation, as will be on some future occasion 
more fully described. This intimate union plays an 
important part, according to Sachs, in the absorption 
of water and of the inorganic matter dissolved in it. 
The mechanical aid afforded by the root-hairs in pene- 
trating the ground is probably only a secondary 
service. 

The tip of the radicle, as soon as it protrudes from 
the seed-coats, begins to circumnutate, and the whole 



* ' Physiologie Vegetate,' 1S6S, pp. 109, 205. 



70 ACTION OF THE EADICLE. Chap. II 

growing part continues to do so, probably for as long 
as growth continues. This movement of the radicle 
has been described in Brassica, iEsculus, Phaseolus, 
Yicia, Cucurbita, Quercus and Zea. The probability 
of its occurrence was inferred by Sachs,* from radicles 
placed vertically upwards being 'acted on by geotro- 
pism (which we likewise found to be the case), for if 
they had remained absolutely perpendicular, the attrac- 
tion of gravity could not have caused them to bend to 
any one side. Circumnutation was observed in the above 
specified cases, either by means of extremely fine fila- 
ments of glass affixed to the radicles in the manner 
previously described, or by their being allowed to 
grow downwards over inclined smoked glass-plates, on 
which they left their tracks. In the latter cases the 
serpentine course (see Figs. 19, 21, 27, 41) showed 
unequivocally that the apex had continually moved 
from side to side. This lateral movement was small 
in extent, being in the case of Phaseolus at most 
about 1 mm. from a medial line to both sides. But 
there was also movement in a vertical plane at right 
angles to the inclined glass-plates. This was shown 
by the tracks often being alternately a little broader 
and narrower, due to the radicles having alternately 
pressed with greater and less force on the plates. 
Occasionally little bridges of soot were left across the 
tracks, showing that the apex had at these spots been 
lifted up. This latter fact was especially apt to occui 



* ' Ueber das Wachsthum der had previously remarked (' Bei 

Wurzeln : Arbeiten des bot. In- tr'age zur Pflanzenphysiologie, 

3tituts in Wiirzburg,' Heft iii. 1868, p. 81) on the fact of radicles 

1873, p. 460. This memoir, be- placed vertically upwards being 

sides its intrinsic and great in- acted on by geotropism, and ho 

rarest, deserves to be studied as a explained it by the supposition 

model of careful investigation, that their growth was not equa] 

and we shall have occasion to on all sides, 
refer to it repeatedly. Dr. Frank 



Ckap. II. ACTION OF THE RADICLE. 71 

when the radicle instead of travelling straight down 
the glass made a semicircular bend ; but Fig. 52 
shows that this may occur when the track is rectilinear. 
The apex by thus rising, was in one instance able to 
surmount a bristle cemented across «an inclined glass- 
plate ; but slips of wood only ^ of an inch in thickness 
always caused the radicles to bend rectangularly to 
one side, so that the apex did not rise to this small 
height in opposition to geotropism. 

In those cases in which radicles with attached fila- 
ments were placed so as to stand up almost vertically, 
they curved downwards through the action of geotro- 
pism, circumnutating at the same time, and their 
courses were consequently zigzag. Sometimes, how- 
ever, they made great circular sweeps, the lines being 
likewise zigzag. 

Eadicles closely surrounded by earth, even when 
this is thoroughly soaked and softened, may perhaps 
be quite prevented from circumnutating. Yet we 
should remember that the circumnutating sheath-like 
cotyledons of Phalaris, the hypocotyls of Solanum, 
and the epicotyls of Asparagus formed round them- 
selves little circular cracks or furrows in a superficial 
layer of damp argillaceous sand. They were also 
able, as well as the hypocotyls of Brassica, to form 
straight furrows in damp sand, whilst circumnutating 
and bending towards a lateral light. In a future 
chapter it will be shown that the rocking or circum- 
nutating movement of the flower-heads of Trifolium 
subterraneum aids them in burying themselves. It is 
therefore probable that the circumnutation of the tip 
of the radicle aids it slightly in penetrating the 
ground ; and it may be observed in several of the 
previously given diagrams, that the movement is 
more strongly pronounced in radicles when they first 
6 



72 ACTION OF THE RADICLE. Chap. II 

protrude from the seed than at a rather later period j 
but whether this is an accidental or an adaptive 
coincidence we do not pretend to decide. Never- 
theless, when young radicles of Phaseolus multiflorvs 
were fixed vertically close over damp sand, in the 
expectation that as soon as they reached it they 
would form circular furrows, this did not occur, — a 
fact which may be accounted for, as we believe, by 
the furrow being filled up as soon as formed by the 
rapid increase of thickness in the apex of the radicle. 
Whether or not a radicle, when surrounded by soft- 
ened earth, is aided in forming a passage for itself 
by circumnutating, this movement can hardly fail 
to be of high importance, by guiding the radicle 
along a line of least resistance, as will be seen in the 
next chapter when we treat of the sensibility of the 
tip to contact. If, however, a radicle in its down- 
ward growth breaks obliquely into any crevice, or a 
hole left by a decayed root, or one made by the 
larva of an insect, and more especially by worms, the 
circumnutating movement of the tip will materially 
aid it in following such open passage ; and we have 
observed that roots commonly run down the old 
burrows of worms.* 

When a radicle is placed in a horizontal or inclined 
position, the terminal growing part, as is well known, 
bends down towards the centre of the earth; and 
Sachs f has shown that whilst thus bending, the growth 
of the lower surface is greatly retarded, whilst that 



* See, also, Prof. Hensen's state- rows made by worms, 

ments (' Zeitschiift fur Wissen, f ' Arbeiten des bot. Inst. 

Zool.,' B. xxviii. p. 354, 1S77) to Wiirzburg,' vol. i. 1873, p. 4b* 1. 

the same effect. He goes so far See also p. 397 for the length of 

as to believe that roots are able the growing part, and p. 451 on 

to penetrate the ground to a great the force of geotropi^m. 
rlepth only by means of the bur- 



Chav. tt. action of the kadicle. 73 

of the upper surface continues at the normal rate, 
or may be even somewhat increased. He has further 
shown by attaching a thread, running over a pulley, 
to a horizontal radicle of large size, namely, that 
of the common bean, that it was able to pull up a 
weight of only one gramme, or 15*4 grains. We may 
therefore conclude that geotropism does not give a 
radicle force sufficient to penetrate the ground, but 
merely tells it (if such an expression may be used) 
which course to pursue. Before we knew of Sachs' 
more precise observations we covered a flat surface of 
damp sand with the thinnest tin-foil which we could 
procure ("02 to '03 mm., or '00012 to -00079 of an inch 
in thickness), and placed a radicle close above, in such 
a position that it grew almost perpendicularly down- 
wards. When the apex came into contact with the 
polished level surface it turned at right angles and 
glided over it without leaving any impression ; yet 
the tin-foil was so flexible, that a little stick of soft 
wood, pointed to the same degree as the end of the 
radicle and gently loaded with a weight of only a 
quarter of an ounce (120 grains) plainly indented the 
tin-foil. 

Kadicles are able to penetrate the ground by the 
force due to their longitudinal and transverse growth ; 
the seeds themselves being held down by the weight 
of the superincumbent soil. In the case of the bean 
the apex, protected by the root-cap, is sharp, and 
the growing part, from 8 to 10 mm. in length, is 
much more rigid, as Sachs has proved, than the part 
immediately above, which has ceased to increase in 
length. We endeavoured to ascertain the downward 
pressure of the growing part, by placing germinating 
beans between two small metal plates, the upper one 
of which was loaded with a known weight: and the 



74 ACTION OF THE KADICLE. Chap. T£ 

radicle was then allowed to grow into a narrow hole iu 
wood, 2 or 3 tenths of an inch in depth, and closed at 
the bottom. The wood was so cut that the short space 
of radicle between the month of the hole and the 
bean could not bend laterally on three sides ; but it 
was impossible to protect the fourth side, close to 
the bean. Consequently, as long as the radicle con- 
tinued to increase in length and remained straight, 
the weighted bean would be lifted up after the tip 
bad reached the bottom of the shallow hole. Beans 
thus arranged, surrounded by damp sand, lifted up a 
quarter of a pound in 24 h. after the tip of the 
radicle had entered the hole. With a greater weight 
the radicles themselves always became bent on the one 
unguarded side; but this probably would not haye 
occurred if they had been closely surrounded on all 
sides by compact earth. There was, however, a 
possible, but not probable, source of error in these 
trials, for it was not ascertained whether the beans 
themselves go on swelling for several days after they 
have germinated, and after having been treated in 
the manner in which ours had been ; 
*_^_. namely, being first left for 24 h. in 



water, then allowed to germinate in 
— J very damp air, afterwards placed over 
autiine of piece of the hole and almost surrounded by 

stick (reduced to d d { j d fe 

one-half natural r 

size) with a hole We succeeded better in ascertaining 
the°mdicie ^of'a tne force exerted transversely by these 
bean grew. Thick- radicles. Two were so placed as to 

ness of stick at , . n i i i ■ tmi 

narrow* end -08 penetrate small holes made in little 

inch, at broad end sticks, one of which was cut into the 

hole'-i inch. shape here exactly copied (Fig. 55). 

The short end of the stick beyond 

the hole was purposely split, but not the opposite 



CUAP. II. 



ACTION OF THE RADICLE. 



76 



Fig. 56. 



end. As the wood was highly elastic, the split 01 
fissure closed immediately after being made. After 
six days the stick and bean were dug out of the damp 
sand, and the radicle was found to be much enlarged 
above and beneath the hole. The fissure, which was 
at first quite closed, was now open to a width of 
4 mm. ; as soon as the radicle was extracted, it imme- 
diately closed to a width of 2 mm. The stick was 
then suspended horizontally by 
a fine wire passing through the 
hole lately filled by the radicle, 
and a little saucer was sus- 
pended beneath to receive the 
weights ; and it required 8 lbs. 
8 ozs. to open the fissure to the 
width of 4 mm. — that is, the 
width before the root was ex- 
tracted. But the part of the 
radicle (only *1 of an inch in 
length) which was embedded in 
-the hole, probably exerted a 
greater transverse strain even 
than 8 lbs. 8 ozs., for it had split 
the solid wood for a length of 
rather more than a quarter of 
an inch (exactly "275 inch), and 
this fissure is shown in Fig. 55. 
A second stick was tried in the 
same manner with almost ex- 
actly the same result. 

We then followed a better 
plan. Holes were bored near 
the narrow end of two wooden clips or pincers (Fig. 56), 
kept closed by brass spiral springs. Two radicles in damp 
«and were allowed to grow through these holes. The 




Wooden pincers, kept closed by 
a spiral brass spring, with a 
hole (*14 inch in diameter 
and "6 inch in depth) bored 
through the narrow closed 
part, through which a radicle 
of a bean was allowed to 
grow. Temp. 50°-60° F. 



76 ACTION OF THE RADICLE. Chap 7T. 

pincers rested on glass-plates to lessen the friction from 
the sand. The holes were a little larger (viz. *14 inch) 
and considerably deeper (viz. *6 inch) than in the 
trials with the sticks ; so that a greater length of a 
rather thicker radicle exerted a transverse strain. 
After 13 days they were taken up. The distance of 
two dots (see the figure) on the longer ends of the 
pincers was now carefully measured ; the radicles were 
then extracted from the holes, and the pincers of 
course closed. They were then suspended horizontally 
in the same manner as were the bits of sticks, and a 
weight of 1500 grams (or 3 lbs. 4 ozs.) was necessary 
with one of the pincers to open them to the same 
extent as had been effected by the transverse growth 
of the radicle. As soon as this radicle had slightly 
opened the pincers, it had grown into a flattened form 
and had escaped a little beyond the hole ; its diameter 
in one direction being 4 - 2 mm., and at right angles 
3*5 mm. If this escape and flattening could have 
been prevented, the radicle would probably have 
exerted a greater strain than the 3 lbs. 4 ozs. With 
the other pincers the radicle escaped still further 
out of the hole ; and the weight required to open 
them to the same extent as had been effected by the 
radicle, was only 600 grams. 

With these facts before us, there seems little diffi- 
culty in understanding how a radicle penetrates the 
ground. The apex is pointed and is protected by 
the root-cap ; the terminal growing part is rigid, and 
increases in length with a force equal,, as far as our 
observations can be trusted, to the pressure of at least 
a quarter of a pound, probably with a much greater 
force when prevented from bending to any side by the 
surrounding earth. Whilst thus increasing in length 
it increases in thickness, pushing away the damp 



CiiAP II. HYPOCOTYLS AND EFICOTYLS. 77 

earth on all sides, with a force of above 8 pounds in 
one case, of 3 pounds in another case. It was impos- 
sible to decide whether the actual apex exerts, relatively 
to its diameter, the same transverse strain as the parts 
a little higher up ; but there seems no reason to doubt 
that this would be the case. The growing part there- 
fore does not act like a nail when hammered into a 
board, but more like a wedge of wood, which whilst 
slowly driven into a crevice continually expands at 
the same time by the absorption of water; and a 
wedge thus acting will split even a mass of rock. 

Manner in which Hypocotyls, Epicotyls, &c, rise up 
and break through the ground. — After the radicle has 
penetrated the ground and fixed the seed, the hypo- 
cotyls of all the dicotyledonous seedlings observed by 
us, which lift their cotyledons above the surface, break 
through the ground in the form of an arch. When 
the cotyledons are hypogean, that is, remain buried in 
the soil, the hypocotyl is hardly developed, and the 
epicotyl or plumule rises in like manner as an arch 
through the ground. In all, or at least in most of such 
cases, the downwardly bent apex remains for a time 
enclosed within the seed-coats. With Corylus avel- 
lena the cotyledons are hypogean, and the epicotyl 
is arched; but in the particular case described in 
the last chapter its apex had been injured, and it 
grew laterally through the soil like a root; and in 
consequence of this it had emitted two secondary 
shoots, which likewise broke through the ground as 
arches. 

Cyclamen does not produce any distinct stem, and 
only a single cotyledon appears at first ; * its petiole 



* This is the conclusion arrived considered by other botflnisfs as 

at by Dr. H. Gressner ('Bot. the first true leaf is really the 

Zeitung,' 1874, p. 837), who second cotyledon, which is greatly 

maintains that what has been delayed in its development. 



78 



HYPOCOTYLS, EPICOTYLS, ETC., Ciiap. it 




Cyclamen 
seedling, 
larged : 



Persicum : 
figure en- 
c, blade of 
cotyledon, not yet 
expanded, with arched 
petiole beginning to 
straighten itself; h, 
hypocotyl developed 
into a corm ; r, second- 
ary radicles. 



bleaks through the ground as an arch (Fig. 57). 
Fig. 57. Abronia also has only a single fully 

developed cotyledon, but in this 
case it is the hypocotyl which first 
emerges and is arched. Abronia 
umbellata, however, presents this 
peculiarity, that the enfolded blade 
of the one developed cotyledon 
(with the enclosed endosperm) 
whilst still beneath the surface has 
its apex upturned and parallel to 
the descending leg of the arched 
hypocotyl ; but it is dragged 
out of the ground by the con- 
tinued growth of the hypocotyl, 
with the apex pointing downward. 
With Cycas pectinata the cotyledons are hypogean, 

and a true leaf first breaks 
through the ground with 
its petiole forming an 
arch. 

In the genus Acanthus 
the cotyledons are likewise 
hypogean. In A. mollis, 
a single leaf first breaks 
through the ground with 
its petiole arched, and with 
the opposite leaf much less 
developed, short, straight, 
of a yellowish colour, and 
with the petiole at first not 
half as thick as that of the 
other. The undeveloped 
leaf is protected by stand- 
ing beneath its arched fel- 
low; and it is an instruc- 




Acanthus mollis: seedling, with the 
hypogean cotyledon on the near 
side removed and the radicles cut 
off: a, blade of first leaf begin- 
ning to expand, with petiole still 
partially arched ; b, second and 
opposite leaf, as yet very imper- 
fectly developed : c, hypogean 
cotvlcdon on the opposite side. 



Ohai>. II. BREAKING THEOUGH THE GKOUND. 79 

five fact that it is not arched, as it has not to force 
for itself a passage through the ground. In the accom- 
panying sketch (Fig. 58) the petiole of the first leaf 
has already partially straightened itself, and the blade 
is beginning to unfold. The small second leaf ulti- 
mately grows to an equal size with the first, but this 
process is effected at very different rates in different 
individuals : in one instance the second leaf did not 
appear fully above the ground until six weeks after the 
first leaf. As the leaves in the whole family of the 
Acanthacese stand either opposite one another or in 
whorls, and as these are of equal size, the great in- 
equality between the first two leaves is a singular fact. 
We can see how this inequality of development and 
the arching of the petiole could have been gradually 
acquired, if they were beneficial to the seedlings by 
favouring their emergence ; for with A. candelabrum, 
sjmwsus, and latifolius there was great variability in the 
inequality between the two first leaves and in the 
arching of their petioles. In one seedling of A. can- 
delabrum the first leaf was arched and nine times as 
long as the second, which latter consisted of a mere 
little, yellowish- white, straight, hairy style. In other 
seedlings the difference in length between the two 
] eaves was as 3 to 2, or as 4 to 3, or as only "76 to 
• 62 inch. In these latter cases the first and taller leaf 
was not properly arched. Lastly, in another seedling- 
there was not the least difference in size between the 
two first leaves, and both of them had their petioles 
straight ; their laminae were enfolded and pressed 
against each other, forming a lance or wedge, by 
which means they had broken through the ground. 
Therefore in different individuals of this same species 
of Acanthus the first pair of leaves breaks through 
the ground by two widely different methods ; and ii 



80 HYPOCOTYLS, EPICOTTLS, ETC., Chap, a 

either had proved decidedly advantageous or disad- 
vantageous, one of them no doubt would soon have 
prevailed. 

Asa Gray has described * the peculiar manner of ger- 
mination of three widely different plants, in which the 
hypocotyl is hardly at all developed. These were there- 
fore observed by us in relation to our present subject. 

Delphinium nudicaule. — The elongated petioles of 
the two cotyledons are confluent (as are sometimes 
their blades at the base), and they break through the 
ground as an arch. They thus resemble in a most 
deceptive manner a hypocotyl. At first they are 
solid, but after a time become tubular ; and the basal 
part beneath the ground is enlarged into a hollow 
chamber, within which the young leaves are developed 
without any prominent plumule. Externally root- 
hairs are formed on the confluent petioles, either a 
little above, or on a level with, the plumule. The 
first leaf at an early period of its growth and whilst 
within the chamber is quite straight, but the petiole 
soon becomes arched; and the swelling of this part 
(and probably of the blade) splits open one side of 
the chamber, and the leaf then emerges. The slit 
was found in one case to be 3*2 mm. in length, and 
it is seated on the line of confluence of the two 
petioles. The leaf when it first escapes from the 
chamber is buried beneath the ground, and now an 
upper part of the petiole near the blade becomes 
arched in the usual manner. The second leaf comes 
out of the slit either straight or somewhat arched, but 
afterwards the upper part of the petiole, — certainly in 
some, and we believe in all cases, — arches itself whilst 
forcing a passage through the soil. 



Botanical Text-Book,' 1879, p. 22. 



Ohap. II. BREAKING THROUGH THE GROUND. 81 

Megarrhiza Calif ornica. — The cotyledons of this 
Gourd never free themselves from the seed-coats and 
are hypogean. Their petioles are completely con- 
fluent, forming a tube which terminates downwards 
in a little solid point, consisting of a minute radicle 
and hypocotyl, with the likewise minute plumule 
enclosed within the base of the tube. This structure 
was well exhibited in an abnormal specimen, in which 
one of the two cotyledons failed to produce a petiole, 
whilst the other produced one consisting of an open 
semicylinder ending in a sharp point, formed of the 
parts just described. As soon as the confluent 
petioles protrude from the seed they bend down, as 
they are strongly geotropic, and penetrate the ground. 
The seed itself retains its original position, either 
on the surface or buried at some depth, as the case 
may be. If, however, the point of the confluent 
petioles meets with some obstacle in the soil, as 
appears to have occurred with the seedlings described 
and figured by Asa Gray,* the cotyledons are lifted 
up above the ground. The petioles are clothed with 
root-hairs like those on a true radicle, and they 
likewise resemble radicles in becoming brown when 
immersed in a solution of permanganate of potassium. 
Our seeds were subjected to a high temperature, and 
in the course of three or four days the petioles pene- 
trated the soil perpendicularly to a depth of from 
2 to 2J inches ; and not until then did the true 
radicle begin to grow. In one specimen which was 
closely observed, the petioles in 7 days after their 
first protrusion attained a length of 2 J inches, and the 
radicle by this time had also become well developed. 
The plumule, still enclosed within the tube, was now 



American Journal of Science,' vol. xiv. 1877, p. 21. 



82 



HYPOCOTYLS, EPICOTYLS, ETC. 



Chap. It 



Fig. 58, A. 



"o* inch in length, and was quite straight ; but from 
having increased in thickness it had just begun to 
split open the lower part of the petioles on one side, 
along the line of their confluence. By the following 
morning the upper part of the plumule had arched 
itself into a right angle, and the 
convex side or elbow had thus been 
forced out through the slit. Here 
then the arching of the plumule 
plays the same part as in the case of 
the petioles of the Delphinium. As 
the plumule continued to grow, the 
tip became more arched, and in 
the course of six days it emerged 
through the 2J inches of superin- 
cumbent soil, still retaining its 
arched form. After reaching the 
surface it straightened itself in the 
usual manner. In the accompany- 
ing figure (Fig. 58, A) we have a 
sketch of a seedling in this ad- 
vanced state of development; the 
surface of the ground being re- 

Meqarrhiza CaKfomica : presented by the line G Gr. 

sketch of seedling, The germination of the seeds in 

copied from Asa dray, © 

reduced to one-half their native Californian home pro- 

S'm' sVtatst;: ceeds in a rather different manner, 

the two confluent as we infer from an interesting 

pocoiyi ; a A nd n radic^; letter from Mr. Rattan, sent to us 

pi, plumule ; G G, \,y p r of. Asa Gray. The petioles 

surface of soil. ,1^*1 i e±. 

protrude from the seeds soon alter 
the autumnal rains, and penetrate the ground, generally 
in a vertical direction, to a depth of from 4 to even 
8 inches. They were found in this state by Mr. 
Rattan during the Christmas vacation, with the pin- 




Chap. II BREAKING THROUGH THE GROUND. 83 

mules still enclosed within the tubes ; and he remarks 
that if the plumules had been at once developed and 
had reached the surface (as occurred with our seeds 
which were exposed to a high temperature), they 
would surely have been killed by the frost. As it is 
they lie dormant at some depth beneath the surface, 
and are thus protected from the cold ; and the root- 
hairs on the petioles would supply them with sufficient 
moisture. We shall hereafter see that many seedlings 
are protected from frost, but by a widely different 
process, namely, by being drawn beneath the surface 
by the contraction of their radicles. We may, how- 
ever, believe that the extraordinary manner of germi- 
nation of Megarrhiza has another and secondary 
advantage. The radicle begins in a few weeks to 
enlarge into a little tuber, which then abounds with 
starch and is only slightly bitter. It would therefore 
be very liable to be devoured by animals, were it not 
protected by being buried whilst young and tender, at a 
depth of some inches beneath the surface. Ultimately 
it grows to a huge size. 

Ipomoea hptophylla. — In most of the species of this 
genus the hypocotyl is well developed, and breaks 
through the ground as an arch. But the seeds of the 
present species in germinating behave like those of 
Megarrhiza, excepting that the elongated petioles of 
the cotyledons are not confluent. After they have 
protruded from the seed, they are united at their 
lower ends with the undeveloped hypocotyl and un- 
developed radicle, which together form a point only 
about '1 inch in length. They are at first highly 
geotropic, and penetrate the ground to a depth of 
rather above half an inch. The radicle then begins 
to grow. On four occasions after the petioles had 
grown for a short distance vertically downwards, the\ 



84 HYPOCOTYLS, EPICOTYLS, ETC., Chap. II. 

were placed in a horizontal position in damp air in the 
dark, and in the course of 4 hours they again became 
curved vertically downwards, having passed through 
90° in this time. But their sensitiveness to geotropism 
lasts for only 2 or 3 days; and the terminal part 
alone, for a length of between *2 and "4 inch, is thus 
sensitive. Although the petioles of our specimens 
did not penetrate the ground to a greater depth than 
about J inch, yet they continued for some time to grow 
rapidly, and finally attained the great length of about 
3 inches. The upper part is apogeotropic, and there- 
fore grows vertically upwards, excepting a short 
portion close to the blades, which at an early period 
bends downwards and becomes arched, and thus 
breaks through the ground. Afterwards this portion 
straightens itself, and the cotyledons then free them- 
selves from the seed-coats. Thus we here have in 
different parts of the same organ widely different kinds 
of movement and of sensitiveness ; for the basal part 
is geotropic, the upper part apogeotropic, and a portion 
near the blades temporarily and spontaneously arches 
itself. The plumule is not developed for -some little 
time ; and as it rises between the bases of the parallel 
and closely approximate petioles of the cotyledons, 
which in breaking through the ground have formed an 
almost open passage, it does not require to be arched and 
is consequently always straight. Whether the plumule 
remains buried and dormant for a time in its native 
country, and is thus protected from the cold of winter, 
we do not know. The radicle, like that of the Megar- 
rhiza, grows into a tuber-like mass, which ultimately 
attains a great size. So it is with Ipomaea pandurata, 
the germination of which, as Asa Gray informs us, 
resembles that of I. leptophylla. 

The following case is interesting in connection with 



Chap. II BREAKING THROUGH THE GROUND. 85 

the root-like nature of the petioles. The radicle of a 
seedling was cut off, as it was completely decayed, 
and the two now separated cotyledons were planted. 
They emitted roots from their bases, and continued 
green and healthy for two months. The blades of 
both then withered, and on removing the earth the 
bases of the petioles '(instead of the radicle) were 
found enlarged into little tubers. Whether these 
would have had the power of producing two in- 
dependent plants in the following summer, we do not 
know. 

In Quercus virens, according to Dr. Engelmann,* 
both the cotyledons and their petioles are confluent. 
The latter grow to a length "of an inch or even 
more;" and, if we understand rightly, penetrate the 
ground, so that they must be geotropic. The nutri- 
ment within the cotyledons is then quickly transferred 
to the hypocotyl or radicle, which thus becomes 
developed into a fusiform tuber. The fact ol 
tubers being formed by the foregoing three widely 
distinct plants, makes us believe that their protection 
from animals at an early age and whilst tender, is one 
at least of the advantages gained by the remark- 
able elongation of the petioles of the cotyledons, 
together with their power of penetrating the ground 
like roots under the guidance of geotropism. 

The following cases may be here given, as they bear 
on our present subject, though not relating to seed- 
lings. The flower-stem of the parasitic Lathrsea 
squamaria, which is destitute of true leaves, breaks 
through the ground as an arch ;f so does the flower- 



* ' Transact. St. Louis Acad. ground cannot fail to be great)} 

Science,' vol. iv. p. 190. facilitated by the extraordinary 

t The passage of the flower- quantity of water secreted at this 

etem of the Lathrcea through the period of the year by the subter* 



86 



HYPOCOTYLS, EPICOTYLS, ETC., Chap. II, 



stein of the parasitic and leafless Monotropa hypopitys. 
With Helleborus niger, the flower-stems, which rise up 
independently of the leaves, likewise break through 
the ground as arches. This is also the case with the 
greatly elongated flower-stems, as well as with the 
petioles of Epimedium pinnatum. So it is with the 
petioles of Ranunculus ficaria, when they have to break 
through the ground, but when they arise from the 
summit of the bulb above ground, they are from the 
first quite straight ; and this is a fact which deserves 
notice. The rachis of the bracken fern (Pteris aqui- 
lina), and of some, probably many, other ferns, like- 
wise rises above ground under the form of an arch. 
No doubt other analogous instances could be found by 
careful search. In all ordinary cases of bulbs, rhizomes, 



ranean scale-like leaves : not that 
there is any reason to suppose 
that the secretion is a special 
adaptation for this purpose : it 
probably follows from the areat 
quantity of sap absorbed in the 
early spring by the parasitic roots. 
After a long period without any 
rain, the earth had become light- 
coloured and very dry, but it was 
dark coloured and damp, even in 
parts quite wet. for a distance of 
al least six inches all round eacli 
flower-stem. The water is secreted* 
by glands (described by Cohn, 
' Bericht. Bot. Sect, der Schle- 
bi>cen Gesell.,' 1876, p. 113) 
which line the longitudinal 
channels running through e ich 
scale-like leaf. A large plant was 
dug up, washed so as to remove 
t'.e earth, left for some time to 
drain, and then placed in the 
evening on a dry glass-plate, 
covered with a bell-glass, and by 
next morning it bad secreted a 
large pool of water. The pLte 
was wiped dry, and in the course 
of the succeeding 7 or 8 hours 



another little pool was secreted, 
and after 16 additional hours 
several large drops. A smaller 
plant was washed and placed in a 
large jar, which was left inclined 
for an hour, by whicli time no 
more water drained off. The jar 
was then placed upright and 
closed : after 23 hours twodrachms 
of water were collected from the 
bottom, and a little more after 25 
additional hours. The flower- 
stems were now cut off, for tbey 
do not secrete, and the subter- 
ranean part of the plant was found 
to weigh 106-8 grams (1611 
grains), and the water seer, ted 
during the 48 . hours weighed 
11 - 9 grams (1^3 grains). — that is, 
one-ninth of the whole weight of 
the plant, excluding the flower- 
stems. We should remember that 
plants in a state of nature would 
probably secrete in 48 hours much 
more than the above large amount, 
for their roots would continue all 
the time absorbing sap from the 
plant on which they were para' 
sitic. 



Chap. II. BREAKING THROUGH THE GROUND. 87 

root-stocks, &c, buried beneath the ground, the surface 
is broken by a cone formed by the young imbricated 
leaves, the combined growth of which gives them force 
sufficient for the purpose. 

With germinating monocotyledonous seeds, of 
which, however, we did not observe a large number, 
the plumules, for instance, those of Asparagus and 
Canna, are straight whilst breaking through the ground. 
With the Graminese, the sheath-like cotyledons are 
likewise straight ; they, however, terminate in a sharp 
crest, which is white and somewhat indurated ; and this 
structure obviously facilitates their emergence from 
the soil : the first true leaves escape from the sheath 
through a slit beneath the chisel-like apex and at 
right angles to it. In the case of the onion (Allium 
ceipa) we again meet with an arch ; the leaf-like coty- 
ledon being abruptly bowed, when it breaks through 
the ground, with the apex still enclosed within the 
seed-coats. The crown of the arch, as previously 
described, is developed into a white conical pro- 
tuberance, which we may safely believe to be a 
special adaptation for this office. 

The fact of so many organs of different kinds. — 
hypocotyls and epicotyls, the petioles of some coty- 
ledons and of some first leaves, the cotyledons of 
the onion, the rachis of some ferns, and some flower- 
stems — being all arched whilst they break through 
the ground, shows how just are Dr. Haberlandt's * 
remarks on the importance of the arch to seedling 
plants. He attributes its chief importance to the 
upper, young, and more tender parts of the hypocotyl 



* • Die Sohutzeinrichtungen in though our observations lead us 

dnr Eutwickelung der Keim- to differ on some points from ihu 

julanze,' 1877. We have learned author. 
Diiich from this interesting essay, 

7 



88 HYPOCOTYLS, EPICOTYLS, ETC., Cuap II. 

or epicotyl, being thus saved from abrasion and 
pressure whilst breaking through the ground. But 
we think that some importance may be attributed to 
the increased force gained by the hypocotyl, epicotyl, 
or other organ by being at first arched ; for both legs of 
the arch increase in length, and both have points of 
resistance as long as the tip remains enclosed within 
the seed-coats ; and thus the crown of the arch is 
pushed up through the earth with twice as much force 
as that which a straight hypocotyl, &c, could exert. 
As soon, however, as the upper end has freed- itself, 
all the work has to be done by the basal leg. In 
the case of the epicotyl of the common bean, the 
basal leg (the apex having freed itself from the seed- 
coats) grew upwards with a force sufficient to lift a, 
thin plate of zinc, loaded with 12 ounces. Two more 
ounces were added, and the 14 ounces were lifted up 
to a very little height, and then the epicotyl yielded 
and bent to one side. 

With respect to the primary cause of the arching 
process, we long thought in the case of many seedlings 
that this might be attributed to the manner in which 
the hypocotyl or epicotyl was packed and curved 
within the seed-coats ; and that the arched shape thus 
acquired was merely retained until the parts in question 
reached the ^surface of the ground. But it is doubtful 
whether this is the whole of the truth in any case. 
For instance, with the common bean, the epicotyl or 
plumule is bowed into an arch whilst breaking through 
the seed-coats as shown in Fig. 59 (p. 92). The 
plumule first protrudes as a solid knob (e in A), which 
after twenty-four hours' growth is seen (e in B) to be 
the crown of an arch. Nevertheless, with several 
beans which germinated in damp air, and had other* 
wise been treated in an unnatural manner, little 



Chav. II. BREAKING THROUGH THE GROUND. 89 

plumules were developed in the axils of the petioles 
of both cotyledons, and these were as perfectly arched 
as the normal plumule ; yet they had not been sub- 
jected to any confinement or pressure, for the seed- 
coats were completely ruptured, and they grew in the 
open air. This proves that the plumule has an innate 
or spontaneous tendency to arch itself. 

In some other cases the hypocotyl or epicotyl pro- 
trudes from the seed at first only slightly bowed ; but 
the bowing afterwards increases independently of any 
constraint. The arch is thus made narrow, with the 
two legs, which are sometimes much elongated, parallel 
and close together, and thus it becomes well fitted 
for breaking through the ground. 

With many kinds of plants, the radicle, whilst still 
enclosed within the seed and likewise after its first pro- 
trusion, lies in a straight line with the future hypocotyl 
and with the longitudinal axis of the cotyledons. This 
is the case with Cucurbita ovifera ; nevertheless, in 
whatever position the seeds were buried, the hypocotyl 
always came up arched in one particular direction. 
Seeds were planted in friable peat at a depth of aboi. t 
an inch in a vertical position, with the end from which 
the radicle protrudes downwards. Therefore all the 
parts occupied the same relative positions which 
they would ultimately hold after the seedlings had 
risen clear above the surface. Notwithstanding this 
fact, the hypocotyl arched itself; and as the arch 
grew upwards through the peat, the buried seeds were 
turned either upside down, or were laid horizontally, 
being afterwards dragged above the ground. Ulti- 
mately the hypocotyl straightened itself in the usual 
manner ; and now after all these movements the 
several parts occupied the same position relatively to 
one another and to the centre of the earth, which they 



90 HYPOCOTYLS, EPICOTYLS, ETC., Chap. IT 

had done when the seeds were first buried. But it may 
be argued in this and other such cases that, as the 
hypocotyl grows up through the soil, the seed will 
almost certainly be tilted to one side ; and then 
from the resistance which it must offer during its 
further elevation, the upper part of the hypocotyl will 
be doubled down and thus become arched. This view 
seems the more probable, because with Ranunculus 
ficaria only the petioles of the leaves which forced 
a passage through the earth were arched ; and not 
those which arose from the summits of the bulbs above 
the ground. Nevertheless, this explanation does not 
apply to the Cucurbita, for when germinating seeds 
were suspended in damp air in various positions by 
pins passing through the cotyledons, fixed to the 
inside of the lids of jars, in which case the hypo- 
cotyls were not subjected to any friction or constraint, 
yet the upper part became spontaneously arched. 
This fact, moreover, proves that it is not the weight 
of the cotyledons which causes the arching. Seeds 
of Helianthus annuus and of two species of Ipomcea 
(those of I. bona nox being for the genus large 
and heavy) were pinned in the samo manner, 
and the hypocotyls became spontaneously arched ; 
the radicles, which had been vertically dependent, 
assumed in consequence a horizontal position. In 
the case of Ipomoea leptophylla it is the petioles of the 
cotyledons which become arched whilst rising through 
the ground ; and this occurred spontaneously when 
the seeds were fixed to the lids of jars. 

It may, however, be suggested with some degree of 
probability that the arching was aboriginally caused 
by mechanical compulsion, owing to the confinement 
of the parts in question within the seed-coats, or to 
friction whilst they were being dragged upwards. Bat 



Chap. II. BREAKING THROUGH THE GROUND. 91 

if this is so, we must admit from the cases just given, 
that a tendency in the upper part of the several 
specified organs to bend downwards and thus to be- 
come arched, has now become with many plants firmly 
inherited. The arching, to whatever cause it may be 
clue, is the result of modified circumnutation, through 
increased growth along the convex side of the part ; 
such growth being only temporary, for the part always 
straightens itself subsequently by increased growth 
along the concave side, as will hereafter be described. 

It is a curious fact that the hypocotyls of some 
plants, which are but little developed and which 
never raise their cotyledons above the ground, never- 
theless inherit a slight tendency to arch themselves, 
although this movement is not of the least use to 
them. We refer to a movement observed by Sachs 
in the hypocotyls of the bean and some other Legumi- 
nosae, and which is shown in the accompanying figure 
(Fig. 59), copied from his Essay.* The hypocotyl 
and radicle at first grow perpendicularly downwards, 
as at A, and then bend, often in the course of 24 hours, 
into the position shown at B. As we shall here- 
after often have to recur to this movement', we will, for 
brevity sake, call it " Sachs' curvature." At first sight 
it might be thought that the altered position of the 
radicle in B was wholly due to the outgrowth of the 
epicotyl (e), the petiole (p) serving as a hinge ; and 
it is probable that this is partly the cause ; bat the 
hypocotyl and upper part of the radicle themselves 
become slightly curved. 

The above movement in the bean was repeatedly 
seen by us ; but our observations were made chiefly on 
l*haseolus multiflorus, the cotyledons of which are like- 



Arheiten <U'a bot. Iustit. Wiirzburg,' vol. i. 1873, p. 108. 



92 



HYPOCOTYLS, EPICOTYLS, ETC. 



Chap. IX 



wise hypogean. Some seedlings with well-developed 
radicles were first immersed in a solution of perman- 
ganate of potassium ; and, judging from the changea 
of colour (though these were not very clearly defined), 
the hypocotyl is about *3 inch in length. Straight, 
thin, black lines of this length were now drawn from 
the bases of the short petioles along the hypocotyls 



F g. 59 




Vici" faba: germinating seeds, suspended in damp air: A, with radicle 
growing perpendicularly downwards ; B, the same bean after 24 hours 
and after the radicle has curved itself; r, radicle; h, short hypocotyl ; 
<?, epicotyi appearing as a knob in A and as an arch in B ; p, petiole of 
the cotyledon, the latter enclosed within the seed-coats. 

of 23 germinating seeds, which were pinned to the 
lids of jars, generally with the hilum downwards, and 
with their radicles pointing to the centre of the 
earth. After an interval of from 2-i to 48 hours the 
black lines on the hypocotyls of 16 out of the 23 
seedlings became distinctly curved, but in very 
various degrees (namely, with radii between 20 and 



Chap. II. BREAKING THROUGH THE GROUND. 03 

80 mm. on Sachs' cyclometer) in the same relative 
direction as shown at B in Fig. 59. As geotropism 
will obviously tend to check this curvature, seven 
seeds were allowed to germinate with proper pre- 
cautions for their growth in a klinostat,* by which 
means geotropism was eliminated. The position of the 
hypocotyls was observed during four successive days, 
and they continued to bend towards the hilum and 
lower surface of the seed. On the fourth day the/ 
were deflected by an average angle of 63° from a lino 
perpendicular to the lower surface, and were therefore 
considerably more curved than the hypocotyl and 
radicle in the bean at B (Fig. 59), though in the same 
relative direction. 

It will, we presume, be admitted that all leguminous 
plants with hypogean cotyledons are descended from 
forms which once raised their cotyledons above the 
ground in the ordinary manner ; and in doing so, it is 
certain that their hypocotyls would have been abruptly 
arched, as in the case of every other dicotyledonous 
plant. This is especially clear in the case of Phaseolus, 
for out of five species, the seedlings of which we 
observed, namely, P. multiflorus, caracalla, vulgaris, 
Hemandesii and Boxburghii (inhabitants of the Old 
and New Worlds), the three last-named species have 
well-developed hypocotyls which break through the 
ground as arches. Now, if we imagine a seedling of 
the common bean or of P. multiflorus, to behave as its 
progenitors once did, the hypocotyl (A, Fig. 59), in 
whatever position the seed may have been buried; 
would become so much arched that the upper part 
would be doubled down parallel to the lower part ; and 



* An instrument devised by on which the plant under observa- 
Suc.s, consisting essentially of a tion is supported : see ' Wiirzburg 
slowly revolving horizontal axis. Arbeiten,' 1879, p. 209. 



94 



RUDIMENTARY COTYLEDONS. 



Chap. 11 



this is exactly the kind of curvature which actually 
occurs in these two plants, though to a much less 
degree. Therefore we can hardly doubt that their 
short hypocotyls have retained by inheritance a ten- 
dency to curve themselves in the same manner as they 
did at a former period, when this movement was highly 
important to them for breaking through the ground, 
though now rendered useless by the cotyledons being 
hypogean. Kudimentary structures are in most cases 
highly variable, and we might expect that rudimentary 
or obsolete actions would be equally so ; and Sachs' 
curvature varies extremely in amount, and sometimes 
altogether fails. This is the sole instance known to 
us of the inheritance, though in a feeble degree, of 
movements which have become superfluous from 
changes which the species has undergone. 



Rudimentary Cotyledons. — A few remarks on this 
subject may be here interpolated. It is well known 
Fi 60 that some dicotyle- 

donous plants produce 
only a single cotyle-. 
don ; for instance, cer- 
tain species of Ranun- 
culus, Corydalis, Chse- 
rophyllum ; and we 
will here endeavour to 
show that the loss of 
one or both cotyle- 
dons is apparently due 
to a store of nutri- 
ment being laid up in 
some other part, as in 
the hypocotyl or one 
of the two cotyledons, or one of the secondary radicles. 





Pilrus aumntium: two young seedlings: 
c, larger cotyledon ; c', smaller cotyle- 
don ; h, thickened hypocotyl ; r, radicle. 
In A the epicotyl is still arched, in B it 
has become erect. 



Chap. II 



RUDIMENTARY COTYLEDONS 



95 



Fig. 61. 



With tlie orange {Citrus avrantium) the cotyledons are 
hypogean, and one is larger than the other, as may 
be seen in A (Fig. 60). In B the inequality is rather 
greater, and the stem has grown between the points 
of insertion of the two petioles, so that they do not 
stand opposite to one another; in another case the 
separation amounted to one-fifth of an inch. The 
smaller cotyledon of one seedling 
was extremely thin, and not half 
the length of the larger one, so that 
it was clearly becoming rudimen- 
tary.* In all these seedlings the 
Jiypocotyl was enlarged or swollen. 
With Abronia umbellata one of 
the cotyledons is quite rudimen- 
tary, as may be seen (c) in Fig. 61. 
In this specimen it consisted of a 
little green flap, -gV^h inch in 
length, destitute of a petiole and 
covered with glands like those on 
the fully developed cotyledon (c). 

At first it Stood Opposite to the Abronia umbellata . seed 

larger cotyledon ; but as the petiole 

of the latter increased in length 

and grew in the same line with 

the hypocotyl (h), the rudiment 

appeared in older seedlings as if 

seated some way down the hypocotyl. With Abronia 

armaria there is a similar rudiment, which in one 




ling twice natural size: 
c, cotyledon ; c', rudi- 
mentary cotyledon ; h, 
enlarged hypocotyl, 
with a heel or projec- 
tion (//) at the lower 
end : r, radicle 



* Tn Pachira aquatica, as de- 
scribed by Mr. R. I. Lynch 
('Journal Linn. Soc. Bot.' vol. 
xvii. 1878, p. 147), one of the 
hypogean cotyledons is of im- 
mense size; the other is small 
ami soon falls off; the pair do not 
always stand opposite. In another 



and very different water-plant, 
Trapa nutans, one of the cotyle- 
dons, filled with farinaceous 
matter, is much larger than the 
other, which is scarcely visible, 
as is stated by Aug. de Candolle, 
' Phvsiologie Veg.' torn. ii. p. 834, 
1832. 



96 KUDIMENTAKY COTYLEDONS. Chap. II 

specimen was only -r^o-th and in another -gVth incli in 
length; it ultimately appeared as if seated halfway 
down the hypocotyl. In both these species the hypo- 
cotyl is so much enlarged, especially at a very early 
age, that it might almost be called a corrn. The lower 
end forms a heel or projection, the use of which will 
hereafter be described. 

In Cyclamen Persicum the hypocotyl, even whilst still 
within the seed, is enlarged into a regular corm,* and 
only a single cotyledon is at first developed (see former 
Fig. 57.) With Ranunculus ficaria two cotyledons are 
never produced, and here one of the secondary radicles 
is developed at an early age into a so-called bulb.f 
Again, certain species of Chserophyllum and Corydalis 
produce only a single cotyledon ;| in the former the 
hypocotyl, and in the latter the radicle is enlarged, 
according to Irmisch, into a bulb. 

In the several foregoing cases one of the cotyledons 
is delayed in its development, or reduced in size, or 
rendered rudimentary, or quite aborted ; but in other 
cases both cotyledons are represented by mere rudi- 
ments. With Opuntia basilar is this is not the case, 
for both cotyledons are thick and large, and the 
hypocotyl shows at first no signs of enlargement ; but 
afterwards, when the cotyledons have withered and dis- 
articulated themselves, it becomes thickened, and from 
its tapering form, together with its smooth, tough, 
brown skin, appears, when ultimately drawn down to 
some depth into the soil, like a root. On the other 



* Dr. H. Gressner, 'Bot. Zei- Vauchers account ('Hist. Phys. 

tung.' 1874, p. 824. des Plan tes d'Europe,' torn i. 1841, 

t Irmisch, 'Beitr'age zur Mor- p. 149) of the germination of the 

phologie der Pflanzen,' 1854, pp. seeds of several species of Cory- 

II, 12; 'Bot. Zeituug,' 1874, p. dalis, that the bulb or tubernile 

805. begins to be formed at an ex- 

X Delpino, 'Rivisia Botanica,' tremely early age. 
1877, p. 21. It is evident from 



Ckm\ II. EUDIMENTARY COTYLEDONS. 97 

band, with several other Cactese, the hypocotyl is from 
the first much enlarged, and both cotyledons are 
almost or quite rudimentary. Thus with Cereus Land- 
hechii two little triangular projections, representing the 
cotyledons, are narrower than the hypocotyl, which is 
pear-shaped, with the point downwards. In Ehipsalis 
cassytha the cotyledons are represented by mere points 
on the enlarged hypocotyl. In Echinocactus viridescens 
the hypocotyl is globular, with two little prominences 
on its summit. In Pilocereus Hoidletii the hypocotyl, 
much swollen in the upper part, is merely notched on 
the summit ; and each side of the notch evidently repre- 
sents a cotyledon. Stapelia sarpedon, a member of the 
very distinct family of the Asclepiadea3, is fleshy like 
a cactus ; and here again the upper part of the flattened 
hypocotyl is much thickened and bears two minute coty- 
ledons, which, measured internally, were only *15 inch 
in length, and in breadth not equal to one-fourth of the 
diameter of the hypocotyl in its narrow axis ; yet these 
minute cotyledons are probably not quite useless, for 
when the hypocotyl breaks through the ground in the 
form of an arch, they are closed or pressed against one 
another, and thus protect the plumule. They after- 
wards open. 

From the several cases now given, which refer to 
widely distinct plants, we may infer that there is some 
close connection between the reduced size of one or 
both cotyledons and the formation, by the enlargement 
of the hypocotyl or of the radicle, of a so-called bulb. 
But it may be asked, did the cotyledons first tend to 
abort, or did a bulb first begin to be formed? As 
all dicotyledons naturally produce two well-developed 
cotyledons, whilst the thickness of the hypocotyl and 
of the radicle differs much in different plants, it seems 
probable that these latter organs first became from 



98 CIECUMNUTATING MOVEMENTS OF Chap. IT. 

some cause thickened — in several instances apparently 
in correlation with the fleshy nature of the mature 
plant — so as to contain a store of nutriment sufficient 
for the seedling, and then that one or both cotyledons, 
from being superfluous, decreased in size. It is not 
surprising that one cotyledon alone should sometimes 
have been thus affected, for with certain plants, for 
instance the cabbage, the cotyledons are at first of 
unequal size, owing apparently to the manner in which 
they are packed within the seed. It does not, how- 
ever, follow from the above connection, that whenever 
a bulb is formed at an early age, one or both coty- 
ledons will necessarily become superfluous, and conse- 
quently more or less rudimentary. Finally, these 
cases offer a good illustration of the principle of com- 
pensation or balancement of growth, or, as Goethe 
expresses it, " in order to spend on one side, Nature 
is forced to economise on the other side." 

Circumnutation and other movements of Hi^ocotyls 
and Eincotyls, whilst still arched and buried beneath 
the ground, and whilst breaking through it. — According 
to the position in which a seed may chance to 
have been buried, the arched hypocotyl or epicotyl 
will begin to protrude in a horizontal, a more or 
less inclined, or in a vertical plane. Except when 
already standing vertically upwards, both legs of the 
arch are acted on from the earliest period by apo- 
geotropism. Consequently they both bend upwards, 
until the arch becomes vertical. During the whole of 
this process, even before the arch has broken through 
the ground, it is continually trying to circumnutate 
to a slight extent ; as it likewise does if it happens at 
first to stand vertically up, — all which cases have 
been observed and described, more or less fully, in 
the last chapter. After the arch has grown to some 



Chap. II HYPOCOTYLS, ETC., WHILST ARGUED. 09 

height upwards, the basal part ceases to circurnmitate, 
>vhilst the upper part continues to do so. 

That an arched hypocotyl or epicotyl, with the two 
legs fixed in the ground, should be able to cir- 
cumnutate, seemed to us, until we had read Prof. 
Wiesner's observations, an inexplicable fact. He has 
shown* in the case of certain seedlings, whose tips 
are bent downwards (or which nutate), that whilst the 
posterior side of the upper or dependent portion grows 
quickest, the anterior and opposite side of the basal 
portion of the same internode grows quickest ; these 
two portions being separated by an indifferent zone, 
where the growth is equal on all sides. There may 
even be more than one indifferent zone in the same 
internode ; and the opposite sides of the parts above 
and below each such zone grow quickest. This pecu- 
liar manner of growth is called by Wiesner "un- 
dulatory nutation." Circumnutation depends on one 
side of an organ growing quickest (probably preceded 
by increased turgescence), and then another side, 
generally almost the opposite one, growing quickest. 
Now if we look at an arch like this f| and suppose 
the whole of one side — we will say the whole convex 
side of both legs — to increase in length, this would 
not cause the arch to bend to either side. But if the 
outer side or surface of the left leg were to increase 
in length the arch would be pushed over to the right, 
and this would be aided by the inner side of the 
right leg increasing in length. If afterwards the 
process were reversed, the arch would be pushed over 
to the opposite or left side, and so on alternately, — ■ 
that is, it would circumnutate. As an arched hypo* 



* 'Die undulireude Nutation Also published separately se« 
dor Internodien,' Akad. der Wis- p. 32. 
senc/t. (Vienna), Jan. 17th, 1878. 



iOO CIRCUMNUTAT1NG MOVEMENTS OF Chap. II 

cotyl, with the two legs fixed in the ground, certainly 
circumnutates, and as it consists of a single internode, 
we may conclude that it grows in the manner de- 
scribed by Wiesner. It may be added, that the crown 
of the arch _ does not grow, or grows very slowly, foi 
it does not increase much in breadth, whilst the arch 
itself increases greatly in height. 

The circumnutating movements of arched hypo- 
cotyls and epicotyls can hardly fail to aid them in 
breaking through the ground, if this be damp and 
soft ; though no doubt their emergence depends 
mainly on the force exerted by their longitudinal 
growth. Although the arch circumnutates only to a 
slight extent and probably with little force, yet it is 
able to move the soil near the surface, though it may 
not be able to do so at a moderate depth. A pot with 
seeds of Solanum palinacantkum, the tall arched hypo- 
cotyls of which had emerged and were growing rather 
slowly, was covered with fine argillaceous sand kept 
damp, and this at first closely surrounded the bases of 
the arches ; but soon a narrow open crack was formed 
round each of them, which could be accounted for 
only by their having pushed away the sand on all 
sides ; for no such cracks surrounded some little sticks 
and pins which had been driven into the sand. It 
has already been stated that the cotyledons of Phalaris 
and Avena, the plumules of Asparagus and the hypo- 
cotyls of Brassica, were likewise able to displace the 
same kind of sand, either whilst simply circumnu- 
tating or whilst bending towards a lateral light. 

As long as an arched hypocotyl or epicotyl remains 
buried beneath the ground, the two legs cannot sepa- 
rate from one another, except to a slight extent from 
the yielding of the soil; but as soon as the arch 
rises above the ground, or at an earlier period if 



Crap. II. HYPOCOTYLS, ETC., WHILST ARCHED. 10] 

(lie pressure of the surrounding earth be artificially 
removed, the arch immediately begins to straighten 
itself. This no doubt is due to growth along the 
whole inner surface of both legs of the arch ; such 
growth being checked or prevented, as long as the two 
legs of the arch are firmly pressed together. When the 
earth is removed all round an arch and the two legs 
are tied together at their bases, the growth on the 
under side of the crown causes it after a time to 
become much flatter and broader than naturally 
occurs. The straightening process consists of a mo- 
dified form of circumnutation, for the lines described 
during this process (as with the hypocotyl of Brassica, 
and the epicotyls of Vicia and Corylus) were often 
plainly zigzag and sometimes looped. After hypo- 
cotyls or epicotyls have emerged from the ground, 
they quickly become perfectly straight. No trace is 
left of their former abrupt curvature, excepting in the 
case of Allium cepa, in which the cotyledon rarely 
becomes quite straight, owing to the protuberance 
developed on the crown of the arch. 

The increased growth along the inner surface of the 
arch which renders it straight, apparently begins in 
the basal leg or that which is united to the radicle ; 
for this leg, as we often ojbserved, is first bowed back- 
wards from the other leg. This movement facilitates 
the withdrawal of the tip of the epicotyl or of the 
cotyledons, as the case may be, from within the seed- 
coats and from the ground. But the cotyledons often 
emerge from the ground still tightly enclosed within 
the seed-coats, which apparently serve to protect them. 
The seed-coats are afterwards ruptured and cast off by 
the swelling of the closely conjoined cotyledons, and not 
by any movement or their separation from one another. 

Nevertheless, in some few cases, especially with the 



102 RUPTURE OF THE SEED-COATS. Crap. II. 

Cucurbitacere, the seed-coats are ruptured by a curious 
contrivance, described by M. Flahault.* A heel or 
peg is developed on one side of the summit of the 
radicle or base of the hypocotyl ; and this holds down 
the lower half of the seed-coats (the radicle being 
fixed into the ground) whilst the continued growth of 
the arched hypocotyl forces up- 
wards the upper half, and tears 
asunder the seed-coats at one end, 
and the cotyledons are then easily 
withdrawn. The accompanying 
figure (Fig. 62) will render this 
description intelligible. Forty- 
one seeds of Cucurhita ovifera 
were laid on friable peat and were 
covered by a layer about an inch 
in thickness, not much pressed 
down, so that the cotyledons in 
being dragged up were subjected 
to very little friction, yet forty of 
them came up naked, the seed- 

Cucurhita ovifera : germi- i i xx i • i • xi x 

nating seed, showing the coats being left buried in the peat. 
heel or peg projecting r rjhis was certainly dueto the action 

on one side from summit ,, , „ -, .. 

of radicle and holding of the peg, for when it was pre- 
down lower tip of seed- ven ted from acting, the cotyledons, 

coats, which have been in xi 

partially ruptured by as we snail presently see, were 
the growth of the arched nfte& up still enclosed in their 

hypocotyl. L 

seed-coats. Ihey were, however, 
cast off in tne course of two or three days by the 
swelling of the cotyledons. Until this occurs light is 
excluded, and the cotyledons cannot decompose car- 
bonic acid; but no one probably would have thought 
that the advantage thus gained by a little earlier cast* 




* 'Bull. Soc. Cot. de France/ torn. xxiv. 1877, p. 201. 



CHAr. II. EUPTUEE OF THE SEED-COATS. 103 

ing off of the seed-coats would be sufficient to account 
for the development of the peg. Yet, according to 
M. Flahault, seedlings which have been prevented 
from casting their seed-coats whilst beneath the 
ground, are inferior to those which have emerged with 
their cotyledons naked and ready to act. 

The peg is developed with extraordinary rapidity ; 
for it could only just be distinguished in two seed- 
lings, having radicles '35 inch in length, but after an 
interval of only 24 hours was well developed in 
both. It is formed, according to Flahault, by the 
enlargement of the layers of the cortical parenchyma 
at the base of the hypocotyl. If, however, we judge 
by the effects of a solution of permanganate of 
potassium, it is developed on the exact line of 
junction between the hypocotyl and radicle; for 
the flat lower surface, as well as the edges, were 
coloured brown like the radicle: whilst the upper 
slightly inclined surface was left uncoloured like the 
hypocotyl, excepting indeed in one out of 33 im- 
mersed seedlings in which a large part of the upper sur- 
face was coloured brown. Secondary roots sometimes 
spring from the lower surface o'f the peg, which thus 
seems in all respects to partake of the nature of the 
radicle. The peg is always developed on the side which 
becomes concave by the arching of the hypocotyl; 
and it would be of no service if it were formed on any 
other side. It is also always developed with the flat 
lower side, which, as just stated, forms a part of the 
radicle, at right angles to it, and in a horizontal plane. 
This fact was clearly shown by burying some of the 
thin flat seeds in the same position as in Fig. 62, 
excepting that they were not laid on their flat broad 
sides, but with one edge downwards. Nine seeds 
were thus planted, and the peg was developed in tha 
8 



104 RUPTURE OF THE SEED-COATS. Chap, JL 

same position, relative] y to the radicle, as in the 
figure; consequently it did not rest on the flat tip 
of the lower half of the seed-coats, but was inserted 
like a wedge between the two tips. As the arched 
hypocotyl grew upwards it tended to draw up the 
whole seed, and the peg necessarily rubbed against 
both tips, but did not hold either down. The result 
was, that the cotyledons of five out of the nine seeds 
thus placed were raised above the ground still enclosed 
within their seed-coats. Four seeds were buried with 
the end from which the radicle protrudes pointing 
vertically downwards, and owing to the peg being 
always developed in the same position, its apex alone 
came into contact with, and rubbed against the tip on 
one side ; the result was, that the cotyledons of all 
four emerged still within their seed-coats. These cases 
show us how the peg acts in co-ordination with the 
position which the flat, thin, broad seeds would almost 
always occupy when naturally sown. When the tip 
of the lower half of the seed-coats was cut off, Flahault 
found (as we did likewise) that the peg could not act, 
since it had nothing to press on, and the cotyledons 
were raised above the ground with their seed-coats not 
cast off. Lastly, nature shows us the use of the peg ; 
for in the one Cucurbitaceous genus known to us, in 
which the cotyledons are hypogean and do not cast 
their seed-coats, namely, Megarrhiza, there is no 
\restige of a peg. This structure seems to be present 
in most of the other genera in the family, judging from 
Fkhault's statements ; we found it well-developed and 
properly acting in Trichosanthes angnina, in which we 
hardly expected to find it, as the cotyledons are some- 
what thick and fleshy. Few cases can be advanced 
of a structure better adapted for a special purpose 
than the present one. 



Chap. II. RUPTURE OF THE SEED-COATS. 105 

With Mimosa pudica the radicle protrudes from a 
email hole in the sharp edge of the seed ; and on its 
summit, where united with the hypocotyl, a transverse 
ridge is developed at an early age, which clearly aids 
in splitting the tough seed-coats ; but it does not aid 
in casting them off, as this is subsequently effected by 
the swelling of the cotyledons after they have been 
raised above the ground. The ridge or heel therefore 
acts rather differently from that of Cucurbita. Its 
lower surface and the edges were coloured brown by 
the permanganate of potassium, but not the upper 
surface. It is a singular fact that after the ridge has 
done its work and has escaped from the seed-coats, 
it is developed into a frill all round the summit of the 
radicle.* 

At the base of the enlarged hypocotyl of Abronia 
umbellata, where it blends into the radicle, there is a 
projection or heel which varies in shape, but its out- 
line is too angular in our former figure (Fig. 61). The 
radicle first protrudes from a small hole at one end of 
the tough, leathery, winged fruit. At this period the 
upper part of the radicle is packed within the fruit 
parallel to the hypocotyl, and the single cotyledon is 
doubled back parallel to the latter. The swelling of 
these three parts, and especially the rapid development 
of the thick heel between the hypocotyl and radicle 
at the point where they are doubled, ruptures the 
tough fruit at the upper end and allows the arched 
hypocotyl to emerge ; and this seems to be the function 
of the heel. A seed was cut out of the fruit and 



* Our attention was called to fit the junction of the radicle and 

this case by a brief statement by hypocotyl. This seed possesses a 

Nobbe in his ' Handbuch der very hard and tough coat, and 

Samenkunde,' 1876. p. 215, where would be likely to require aid in 

a figure is also given of a seedling bursting and freeing the cotyle- 

of Martvnia with a heel or ridge dons. 



/ 06 RUPTUKE OF THE SEED-COATS. Chap. IL 

allowed to germinate in damp air, and now a thin 
flat disc was developed all round the base of the 
hypocotyl and grew to an extraordinary breadth, like 
the frill described under Mimosa, but somewhat broader. 
Flahault says that with Mirabilis, a member of the 
same family with Abronia, a heel or collar is developed 
all round the base of the hypocotyl, but more on one 
side than on the other; and that it frees the coty- 
ledons from their seed-coats. We observed only old 
seeds, and these were ruptured by the absorption of 
moisture, independently of any aid from the heel and 
before the protrusion of the radicle ; but it does not 
follow from our experience that fresh and tough fruits 
would behave in a like manner. 

In concluding this section of the present chapter it 
may be convenient to summarise, under the form of an 
illustration, the usual movements of the hypocotyls 
and epicotyls of seedlings, whilst breaking through the 
ground and immediately afterwards. We may suppose 
a man to be thrown down on his hands and knees, and 
at the same time to one side, by a load of hay falling 
on him. He would first endeavour to get his arched 
back upright, wriggling at the same time in all 
directions to free himself a little from the surrounding 
pressure ; and this may represent the combined effects 
of apogeotropism and circumnutation, when a seed is so 
buried that the arched hypocotyl or epicotyl protrudes 
at first in 'a horizontal or inclined plane. The man, 
still wriggling, would then raise his arched back as 
high as he could ; and this may represent the growth 
and continued circumnutation of an arched hypocotyl 
or epicotyl, before it has reached the surface of the 
ground. As soon as the man felt himself at all free, he 
would raise the upper part of his body, whilst still on 



Chat. II. CIRCUMNUTATION OF HYPOCOTYLS, ETC. 107 

Lis knees and still wriggling ; and this may represent 
the bowing backwards of the basal leg of the arch, 
which in most cases aids in the withdrawal of the 
cotyledons from the buried and ruptured seed-coats, 
and the subsequent straightening of the whole hypo- 
cotyl or epicotyl — circumnutation still continuing. 

Circumnutatiow of Hijfocotyh and Epicotyls, when 
erect. — The hypocotyls, epicotyls, and first shoots of the 
many seedlings observed by us, after they had become 
straight and erect, circumnutated continuously. The 
diversified figures described by them, often during two 
successive clays, have been shown in the woodcuts in 
the last chapter. It should be recollected that the 
dots were joined by straight lines, so that the figures 
are angular; but if the observations had been made 
every few minutes the lines would have been more 
or less curvilinear, and irregular ellipses or ovals, or 
perhaps occasionally circles, would have been formed. 
The direction of the longer axes of the ellipses made 
during the same day or on successive days generally 
changed completely, so as to stand at right angles to 
one another. The number of ^irregular ellipses or 
circles made within a given time differs much with 
different species. Thus with Brassica oleracea, Cerinthe 
major, and Cueurhita ovifera about four such figures 
were completed in 12 h. ; whereas with Solatium palina- 
canthum and Opuntia basilaris, scarcely more than one. 
The figures likewise differ greatly in size ; thus they 
were very small and in some degree doubtful in Stapelia, 
and large in Brassica, &c. The ellipses described by 
Lathjrus nissolia and Brassica were narrow, whilst 
those made by the Oak were broad. The figures are 
often complicated by small loops and zigzag lines. 

As most seedling plants before the development 
of true leaves are of low, sometimes very low stature, 



108 CIECUMNUTATION OF HYPOCOTYLS, ETC. Chap. IL 

the extreme amount of movement from side to side 
of their circumnutating stems was small; that of 
the hypocotyl of Githago segetum was about *2 of an 
inch, and that of Cucurbita ovifera about '28. A 
very young shoot of Lathy rus nissolia moved about 
•14, that of an American oak *2, that of the common 
nut only *04, and a rather tall shoot of the Asparagus 
•11 of. an inch. The extreme amount of movement 
of the sheath-like cotyledon of Phalaris Canariensis 
was '3 of an inch ; but it did not move very quickly, 
the tip crossing on one occasion five divisions of the 
micrometer, that is, xJo* n °f an incn > in 22 m. 5 s. A 
seedling Nolana prostrata travelled the same distance 
in 10 m. 38 s. Seedling cabbages circumutated much 
more quickly, for the tip of a cotyledon crossed 
T <j -th of an inch on the micrometer in 3 m. 20 s. ; and 
this rapid movement, accompanied by incessant oscil- 
lations, was a wonderful spectacle when beheld under 
the microscope. 

The absence of light, for at least a clay, does not 
interfere in the least with the circumnutation of the 
hypocotyls, epicotyls, or young shoots of the various 
dicotyledonous seedlings observed by us ; nor with that 
of the young shoots of some monocotyledons. The 
circumnutation was indeed much plainer in darkness 
than in light, for if the light was at all lateral the 
stem bent towards it in a more or less zigzag course. 

Finally, the hypocotyls of many seedlings are drawn 
during the winter into the ground, or even beneath it 
so that they disappear. This remarkable process, 
which apparently serves for their protection, has 
been fully described by De Vries.* He shows that 



* ' Bot. Zeitung,' 1879, p. 649. burg,' Jahrg. xvi. p. 16, as quoted 
See also Winkler in ' Verhandl. by Haberlandt, ' Schutzeinriohuu- 
des Bot. Veieins der P. Brandon- gen der Keimpflanze,' 1877, p. 52 






Chap. II. CIRCUMNUTATION OF COTYLEDONS. 109 

it is effected by the contraction of the parenchyma- 
cells of the root. But the hypocotyl itself in some 
cases contracts greatly, and although at first smooth 
becomes covered with zigzag ridges, as we observed 
with Githago segetum. How much of the drawing 
down and burying of the hypocotyl oiOjpuntia hasilaris 
was due to the contraction of this part and how much 
to that of the radicle, we did not observe. 

Circumnutation of Cotyledons. — With all the dico- 
tyledonous seedlings described in the last chapter, the 
cotyledons were in constant movement, chiefly in a ver- 
tical plane, and commonly once up and once down in 
the course of the 24 hours. But there were many excep- 
tions to such simplicity of movement ; thus the cotyle- 
dons of Ipomoea cserulea moved 13 times either upwards 
or downwards in the course of 16 h. 18 m. Those of 
Oxalis rosea moved in the same manner 7 times in the 
course of 24 h. ; and those of Cassia tora described 5 
irregular ellipses in 9 h. The cotyledons of some 
individuals of Mimosa pudica and of Lotus Jacohseus 
moved only once up and clown in 24 h., whilst those of 
others performed within the same period an additional 
small oscillation. Thus with different species, and 
with different individuals of the same species, there 
were many gradations from a single diurnal move- 
ment to oscillations as complex as those of the 
Ipomcea and Cassia. The opposite cotyledons on the 
same seedling move to a certain extent independently 
of one another. This was conspicuous with those of 
Oxalis sensitiva, in which one cotyledon might be 
seen during the daytime rising up until it stood 
vertically, whilst the opposite one was sinking down. 

Although the movements of cotyledons were gene- 
rally in nearly the same vertical plane, yet their 
upward and downward courses never exactly coin* 



110 CIECUMXTJTATIOX OF COTYLEDONS. Chap. II 

cided ; so that ellipses, more or less narrow, were 
described, and the cotyledons may safely be said tc 
have circnmnntated. Nor could this fact be accounted 
for by the mere increase in length of the cotyledons 
through growth, for this by itself would not induce 
any lateral movement. That there was lateral rnove- 
ment in some instances, as with the cotyledons of the 
cabbage, was evident ; for these, besides moving up 
and down, changed their course from right to left 12 
times in 14 h. 15 m. With Solanum lycopersicum the 
cotyledons, after falling in the forenoon, zigzagged 
from side to side between 12 and 4 P.M., and then 
commenced rising. The cotyledons of Lupinus luteus 
are so thick (about '08 of an inch) and neshy,* that 
they seemed little likely to move, and were there- 
fore observed with especial interest; they certainly 
moved largely up and down, and as the line traced was 
zigzag there was some lateral movement. The nine 
cotyledons of a seedling Pinus pinaster plainly circum- 
nutated ; and the figures described approached more 
nearly to irregular circles than to irregular ovals or 
ellipses. The sheath-like cotyledons of the Gra- 
minese circuinnutate, that is, move to all sides, as 
plainly as do the hypocotyls or epicotyls of any dico- 
tyledonous plants. Lastly, the very young fronds of 
a Fern and of a Selaginella circumnutated. 

In a large majority of the cases which were care- 
fully observed, the cotyledons sink a little downwards 
in the forenoon, and rise a little in the afternoon or 
evening. They thus stand rather more highly inclined 
during the night than during the mid-day, at which 

* The cotyledons, though bright & c , 1S77, p. 95), on the gradations 
green, resemble to a certain ex- in the Legun.inosae between sub- 
tent hypogean ones; see the in- aerial and subterranean cotvle 
fceresting discussion by Haber- dons, 
landt ('Die Schutzcinriehtungen,' 



Caap. II. CIRCUMNUTATION OF COTYLEDONS. Ill 

time they are expanded almost horizontally. The 
circumnutating movement is thus at least partially 
periodic, no doubt in connection, as we shall hereafter 
see, with the daily alternations of light and daikuess. 
The cotyledons of several plants move up so much at 
night as to stand nearly or quite vertically; and in 
this latter case ihey come into close contact with one 
another. On the other hand, the cotyledons of a 
few plants sink almost or quite vertically down at 
night ; and in this latter case they clasp the upper 
part of the hypocotyl. In the same genus Oxalis the 
cotyledons of certain species stand vertically up, and 
those of other species vertically down, at night. In 
all such cases the cotyledons may be said to sleep, 
for they act in the same manner as do the leaves of 
many sleeping plants. This is a movement for a 
special purpose, and will therefore be considered in a 
future chapter devoted to this subject. 

In order to gain some rude notion of the proportional 
number of cases in which the cotyledons of dico- 
tyledonous plants (hypogean ones being of course 
excluded) changed their position in a conspicuous 
manner at night, one or more species in several 
genera were cursorily observed, besides those described 
in the last chapter. Altogether 153 genera, included 
in as many families as could be procured, were thus 
observed by us. The cotyledons were looked at in 
the middle of the day and again at night ; and those 
were noted as sleeping which stood either vertically 
or at an angle of at least 60 ° above or beneath the 
horizon. Of such genera there were 26 ; and in 21 of 
them the cotyledons of some of the species rose, and 
in only 6 sank at night ; and some of these latter 
cases are rather doubtful from causes to be explained 
in the chapter on the sleep of cotyledons. When 



112 PULVINI OF COTYLEDONS. Chap. II 

cotyledons which at noon were nearly horizontal, stood 
at ni^ht at more than 20° and less than 60° above the 
horizon, they were recorded, as " plainly raised ;" and 
of such genera there were 38. We did not meet with 
any distinct instances of cotyledons periodically sink- 
ing only a few degrees at night, although no doubt 
such occur. We have now accounted for 64 genera 
out of the 153, and there remain 89 in which the 
cotyledons did not change their position at night by 
as much as 20°— that is, in a conspicuous manner 
which could easily be detected by the unaided eye and 
by memory ; but it must not be inferred from this 
statement that these cotyledons did not move at all, 
for in several cases a rise of a few degrees was re- 
corded, when they were carefully observed. The 
number 89 might have been a little increased, for the 
cotyledons remained almost horizontal at night in 
some species in a few genera, for instance, Trifo- 
lium and Geranium, which are included amongst the 
sleepers, such genera might therefore have been added 
to the 89. Again, one species of Oxalis generally 
raised its cotyledons at night more than 20° and less 
than 60° above the horizon ; so that this genus might 
have been included under two heads. But as several 
species in the same genus were not often observed, 
such double entries have been avoided. 

In a future chapter it will be shown that the leaves 
of many plants which do not sleep, rise a few degrees in 
the evening and during the early part of the night ; 
and it will be convenient to defer until then the 
consideration of the periodicity of the movements of 
cotyledons. 

On the Pulvini or Joints of Cotyledons. — With several 
of the seedlings described in this and the last chapter, 
the summit of the petiole is developed into a pulvinus, 



Chap. II. 



PULVINI OF COTYLEDONS. 



113 



cushion, or joint (as this organ has been variously 
called), like that with which many leaves are provided. 
It consists of a mass of small cells usually of a pale 
colour from the absence of chlorophyll, and with its 
outline more or less convex, as shown in the annexed 
figure. In the case of Oxalis 
sensitiva two-thirds of the 
petiole, and in that of Mi- 
mosa pudica, apparently the 
whole of the short sub- 
petioles of the leaflets have 
been converted into pulvini. 
With pulvinated leaves (i.e. 
those provided with a pul- 
vinus) their periodical move- 
ments depend, according to 
Pfeffer,* on the cells of the 
pulvinus alternately expand- 
ing more quickly on one side 
than on the other; whereas 
the similar movements of 
leaves not provided with pul- 
vini, depend on their growth 
being alternately more rapid 
on one side than on the 
other.f As long as a leaf 
provided with a pulvinus is 
young and continues to grow, 
its movement depends on both these causes combined ;J 
and if the view now held by many botanists be sound, 
namely, that growth is always preceded by the expan- 
sion of the growing cells, then the difference between 




Oxalis rosea : longitudinal section 
of a pulvinus on the summit 
of the petiole of a cotyledon, 
drawn with the camera lucida, 
magnified 75 times : p, p, pe- 

. tiole ; /, fibro-yascular bundle ; 
b, b, commencement of blade ol 
cotyledon. 



Die Periodische Bewegun- 
gcn der BLittorgane,' 1875. 



t Batalin, 'Flora,' Oct. 1st, 1873 
% Pfeffer, ibid. p. 5. 



Ill PULVINI OF COTYLEDONS. Chap. IL 

without such aid, is reduced to the expansion of the 
cells not being followed by growth in the first case, 
and being so followed in the second case. 

Dots were made with Indian ink along the midrib 
of both pulvinated cotyledons of a rather old seedling 
of Oxalis Valdiviana ; their distances were repeatedly 
measured with an eye-piece micrometer during 8f days, 
and they did not exhibit the least trace of increase. 
It is therefore almost certain that the pulvinus itself 
was not then growing. Nevertheless, during this 
whole time and for ten days afterwards, these coty- 
ledons rose vertically every night. In the case of 
some seedlings raised from seeds purchased under the 
name of Oxalis Jforibunda, the cotyledons continued 
for a long time to move vertically clown at night, and 
the movement apparently depended exclusively on 
the pulvini, for their petioles were of nearly the same 
length in young, and in old seedlings which had pro- 
duced true leaves. With some species of Cassia, on 
the other hand, it was obvious without any measure- 
ment that the pulvinated cotyledons continued to 
increase greatly in length during some weeks ; so that 
here the expansion of the cells of the pulvini and the 
growth of the petiole were probably combined m 
causing their prolonged periodic movements. It was 
equally evident that the cotyledons of many plants, 
not provided with pulvini, increased rapidly in length ; 
and their periodic movements no doubt were exclu- 
sively due to growth. 

In accordance with the view that the periodic 
movements of all cotyledons depend primarily on the 
expansion of the cells, whether or not followed by 
growth, we can understand the fact that there is but 
little difference in the kind or form of movement 
in the two sets of cases. This may be seen by com- 



CtoAP. II. PULVINI OF COIYLEDONS. 115 

paring the diagrams given in the last chapter. Thus 
the movements of the cotyledons of Brassica oleraeea 
and of Ipomoea cserulea, which are not provided with 
pulvini, are as complex as those of Oxalis and Cassia 
which are thus provided. The pulvinated cotyledons 
of some individuals of Mimosa pudica and Lotus 
Jacobseus made only a single oscillation, whilst those 
of other individuals moved twice up and down in the 
course of 24 hours ; so it was occasionally with the 
cotyledons of Cucurbita ovifera, which are destitute of 
a pulvinus. The movements of pulvinated cotyledons 
are generally larger in extent than those without a 
pulvinus; nevertheless some of the latter moved 
through an angle of 90°. There is, however, one 
important difference in the two sets of cases; the 
nocturnal movements of cotyledons without pulvini, 
for instance, those in the Cruciferse, Cucurbitacese, 
Githago, and Beta, never last even for a week, to any 
conspicuous degree. Pulvinated cotyledons, on the 
other hand, continue to rise at night for a much 
longer period, even for more than a month, as we 
shall now show. But the period no doubt depends 
largely on the temperature to which the seedlings are 
exposed and their consequent rate of development. 

Oxalis Valdioiana. — Some cotyledons which had late! y opened 
and were horizontal on March 6th at noon, stood at night ver- 
tically up ; on the 13th the first true leaf was formed, and was 
embraced at night by the cotyledons; on April 9th, after an in- 
terval of 35 days, six leaves were developed, and yet the coty- 
ledons rose almost vertically at night. The cotyledons of 
another seedling, which when first observed had already pro- 
duced a leaf, stood vertically at night and continued to do so for 
11 additional days. After 16 days from the first observation 
two leaves were developed, and the cotyledons were still greatly 
raised at night. After 21 days the cotyledons during the day 
were deflected beneath the horizon, but at night were raised 4 j° 



L1G PULVINI OF COTYLEDONS. Chap. IL 

above it. After 24 days from the first observation (begun after 
a true leaf had been developed) the cotyledons ceased to rise at 
night. 

Oxalis (Biophytum) sensitiva. — The cotyledons of several seed- 
lings, 45 days after their first expansion, stood nearly vertical at 
night, and closely embraced either one or two true leaves which 
by this time had been formed. These seedlings had been kept 
in a very warm house, and their development had been rapid. 

Oxalis corniculuta.—Tke cotyledons do not stand vertical at 
night, but generally rise to an angle of about 45° above the 
horizon. They continued thus to act for 23 days after their 
first expansion, by which time two leaves had been formed ; 
even after 29 days they still rose moderately above their hori- 
zontal or downwardly deflected diurnal position. 

Mimosa pudica, — The cotyledons were expanded for the first 
time on Nov. 2nd, and stood vertical at night. On the 15th the 
first leaf was formed, and at night the cotyledons were vertical. 
On the 28th they behaved in the same manner. On Dec. 15th, 
that is after 41 days, the cotyledons were still considerably 
raised at night; but those of another seedling, only one day 
older, were raised very little. 

Mimosa albida. — A seedling was observed during only 12 days, 
by which time a leaf had been formed, and the cotyledons were 
then quite vertical at night. 

Trifolium subterranpum. — A seedling, 8 days old, had its coty- 
ledons horizontal at 10.30 a.m. and vertical at 9.15 p.m. After an 
interval of two months, by which time the first and second true 
leaves had been developed, the cotyledons still performed the 
same movement. They had now increased greatly in size, and 
had become oval; and their petioles were actually "8 of an inch 
in length ! 

Trifolium strict um. — After 17 days the cotyledons still rose at 
night, but were not afterwards observed. 

Lotus Jacobceus. — The cotyledons of some seedlings having 
well-developed leaves rose to an angle of about 45° at night; 
and even after 3 or 4 whorls of leaves had been formed, the co- 
tyledons rose at night considerably above their diurnal hori- 
zontal position. 

Cassia mimosoides. — The cotyledons of this Indian species, 
14 days after their first expansion, and when a leaf had been 
formed, stood during the day horizontal, and at night vertical. 

Cassia sp ? (a large S. Brazilian tree raised from seeds sent us 



Chap. II. PULVINI OF COTYLEDONS. 117 

by F. Miiller).— The cotyledons, after 16 days from their first 
expansion, had increased greatly in size with two leaves just 
formed. They stood horizontally during the day and vertically 
at night, but were not afterwards observed. 

Cassia neglecta (likewise a S. Brazilian species). — A seedling, 
34 days after the first expansion of its cotyledons, was between 3 
and 4 inches in height, with 3 well-developed leaves ; and the 
cotyledons, which during the day were nearly horizontal, at night 
stood vertical, closely embracing the young stem. The cotyle- 
dons of another seedling of the same age, 5 inches in height, 
with 4 well-developed leaves, behaved at night in exactly the 
same manner. 

It is known * that there is no difference in structure 
between the upper and lower halves of the pulvini of 
leaves, sufficient to account for their upward or down- 
ward movements. In this respect cotyledons offer an 
unusally good opportunity for comparing the structure 
of the two halves ; for the cotyledons of Oxalis Valdi- 
viana rise vertically at night, whilst those of 0. rosea 
sink vertically ; yet when sections of their pulvini were 
made, no clear difference could be detected between the 
corresponding halves of this organ in the two species 
which move so differently. With. 0. rosea, however, 
there were rather more cells in the lower than in the 
upper half, but this was likewise the case in one speci- 
men of 0. Valdiviana. The cotyledons of both species 
(3J mm. in length) were examined in the morning 
whilst extended horizontally, and the upper surface of 
the pulvinus of 0. rosea was then wrinkled transversely, 
showing that it was in a state of compression, and this 
might have been expected as the cotyledons sink at 
night ; with 0. Valdiviana it was the lower surface 
which was wrinkled, and its cotyledons rise at night. 

Trifolium is a natural genus, and the leaves of all 



* Pfeffer, * Die Period. Bewegungeri,' 1S75, p. 157. 



1.18 TULVINI OF COTYLEDONS. Chap. II 

the species seen by us are pulvinated ; so it is with 
the cotyledons of T. subterraneum and strictum, which 
stand vertically at night ; whereas those of T. resupi- 
natum exhibit not a trace of a pulvinus, nor of any 
nocturnal movement. This was ascertained by mea- 
suring the distance between the tips of the cotyledons 
of four seedlings at mid-day and at night. In this 
species, however, as in the others, the first-formed leaf, 
which is simple or not trifoliate, rises up and sleeps 
like the terminal leaflet on a mature plant. 

In another natural genus, Oxalis, the cotyledons of 
0. Valdiviana, rosea, floribunda, articulata, and sensitiva 
are pulvinated, and all move at night into an upward 
or downward vertical position. In these several species 
the pulvinus is seated close to the blade of the coty- 
ledon, as is the usual rule with most plants. Oxalis cor- 
niadata (var. Atro-pwpiirea) differs in several respects ; 
the cotyledons rise at night to a very variable amount, 
rarely more than 45°; and in one lot of seedlings 
(purchased under the name of 0. tropseoloides, but 
certainly belonging to the above variety) they rose 
only from 5° to 15° above the horizon. The pulvinus 
is developed imperfectly and to an extremely variable 
degree, so that apparently it is tending towards abor- 
tion. No such case has hitherto, we belie ye, been 
described. It is coloured green from its cells con- 
taining chlorophyll ; and it is seated nearly in the 
middle of the petiole, instead of at the upper end as 
in all the other species. The nocturnal movement is 
effected partly by its aid, and partly by the growth of 
the upper part of the petiole as in the case of plants 
destitute of a pulvinus. Fr^m these several reasons 
and from our having partially traced the develop- 
ment of the pulvinus from an early age, the case 
seems worth describing in some detail. 



Chap. II. 



PULVINI OF COTYLEDONS. 



119 



When the cotyledons of 0. comimluia were dissected out of a 
seed from which they would soon have naturally emerged, no 
trace of a pulvinus could be detected ; and all the cells forming 
the short petiole, 7 in number in a longitudinal row, were of nearly 
equal size. In seedlings one or two days old, the pulvinus was 
so indistinct that we thought at first that it did not exist ; but 
in the middle of the petiole an ill-defined transverse zone of cells 
could be seen, which were much shorter than those both above 
and below, although of the same breadth with them. They 
presented the appearance of having been just formed by the 
transverse division of longer cells ; and there can be little doubt 
that this had occurred, for the cells in the petiole which had 



Fig. 6*. 





Oxalts cornlculata: A and B the almost rudimentary pulvini of the coty- 
ledons of two rather old seedlings, viewed as transparent objects. 
Magnified 50 times. 

been dissected out of the seed averaged in length 7 divisions 
of the micrometer (each division equalling -003 mm ), and were 
a little longer than those forming a well-developed pulvinus, 
which varied between 4 and 6 of these same divisions. After a 
fsw additional days the ill -defined zone of cells becomes distinct, 
and although it does not extend across the whole width of the 
petiole, and although the cells are of a green colour from contain- 
ing chlorophyll, yet they certainly constitute a pulvinus, which, 
as we shall presently see, acts as one. These small cells were 
arranged in longitudinal rows, and varied from 4 to 7 in number ; 
and the cells themselves varied in length in different parts of the 
9 



120 PULVIXI OF COTYLEDONS. Chap. II 

sante pulvinus and in different individuals. In the accompany- 
ing figures, A and B (Fig. 64), we have views of the epidermis * 
in the middle part of the petioles of two seedlings, in which the 
pulvimis was for this species well developed. They offer a 
striking contrast with the pulvhms of 0. rosea (see former 
Fig. 63), or of 0. Valdiviana. With the seedlings, falsely called 
0. trojjceoloides, the cotyledons of which rise very little at night, 
the small cells were still fewer in number and in parts formed 
a single transverse row, and in other parts short longitudinal 
rows of only two or three. Nevertheless they sufficed to attract 
the eye, when the whole petiole was viewed as a transparent 
object beneath the microscope. In these seedlings there could 
hardly be a doubt that the pulvinus was becoming rudimentary 
and tending to disappear; and this accounts for its great 
variability in structure and function. 

In the following Table some measurements of the cells in 
fairly well-developed pulvini of 0. corniculata are given : — 

Seedling 1 day old, v:ith cotyledon 23 mm. in length. 

Divisions of 
Micrometer.f 

Average length of cells of pulvinus 6 to 7 

Length of longest cell below the pulvinus 13 

Length of longest cell above the pulvinus 20 

Seedling 5 d ys old, cotyledon 3*1 mm. in length, wi.h the pulvinus 
quite distinct. 

Average length of cells of pulvinus 6 

Length of longest cell below the pulvinus 22 

Leng;h of longest cell above the pulvinus 40 

Seedling 8 days o 7 d, cotyledon 5 mm. in length, with a true leaf 
formed but not yet expanded. 

Average length of cells of pulvinus 9 

Length of longest cell below the pulvinus 44 

Length of longest cell above the pulvinus 70 

Seedling 13 days old, cotyledon 4-5 mm. in length, with a small 
true leaf fully developed. 

Average length of cells of pulvinus 7 

Length of longest cell below the pulvinus .. .. .. 30 

Length of longest cell above the pulvinus t0 



* Longitudinal sections show pulvinus. 
that the forms of the epidermic f Each division equalled '0011 

cells may be taken as a fair repre- mm. 
Bcntation of those constituting the 



Chap. II. PULVINI OF COTYLEDONS 121 

We here see that the cells of the pulviims increase but little 
in length with advancing age, in comparison with those of the 
petiole both above and below it ; but they continue to grow in 
width, and keep equal in this respect with the other cells of 
the petiole. The rate of growth, however, varies in all parts 
of the cotyledons, as may be observed in the measurements of 
the 8-days' old seedling. 

The cotyledons of seedlings only a day old rise at night con- 
siderably, sometimes as much as afterwards; but there was 
much variation in this respect. As the pulvinus is so indistinct 
at first, the movement probably does not then depend on the 
expansion of its cells, but on periodically unequal growth in 
the petiole. By the comparison of seedlings of different known 
ages, it was evident that the chief seat of growth of the petiole 
was in the upper part between the pulvinus and the blade ; 
and this agrees with the fact (shown in the measurements above 
given) that the cells grow to a greater length in the upper than 
in the lower part. With a seedling 1 1 days old, the nocturnal 
rise was found to depend largely on the action of the pulvinus, 
for the petiole at night was curved upwards at this point ; and 
during the day, whilst the petiole was horizontal, the lower 
surface of the pulvinus was wrinkled with the upper surface 
tense. Although the cotyledons at an advanced age do not rise 
at night to a higher inclination than whilst young, yet they have 
to pass through a larger angle (in one instance amounting to 
(53°) to gain their nocturnal position, as they are generally 
deflected beneath the horizon during the day. Even with the 
11-days' old seedling the movement did not depend exclusively 
on the pulvinus, for the blade where joined to the petiole was 
curved upwards, and this must be attributed to unequal growth. 
Therefore the periodic movements of the cotyledons of 0. corni- 
culata depend on two distinct but conjoint actions, namely, the 
expansion of the cells of the pulvinus and on the growth of 
the upper part of the petiole, including the base of the blade. 

Lotus Jacobceus. : — The seedlings of this plant present a case 
parallel to that of Oxalis corniculata in some respects, and in 
others unique, as far as we have seen. The cotyledons during 
the first 4 or 5 days of their life do not exhibit any plain noc- 
turnal movement ; but afterwards they stand vertically or 
almost vertically up at night. There is, however, some degree of 
variability in this respect, apparently dependent on the season 
and on the degree to which they have been illuminated during 



122 PULVINI OF COTYLEDONS. Chap. IL 

Llie day. With older seedlings, having cotyledons 4 mm. in 
length, which rise considerably at night, there is a well -deve- 
loped pulvinus close to the blade, colourless, and rather nar- 
rower than the rest of the petiole, from which it is abruptly 
separated. It is formed of a mass of small cells of an average 
length of -021 mm. ; whereas the cells in the lower part of the 
petiole are about *06 mm., and those in the blade from # 034 to 
•04 mm. in length. The epidermic cells in the lower part of the 
petiole project conically, and thus differ in shape from those 
over the pulvinus. 

Turning now to very young seedlings, the cotyledons of which 
do not rise at night and are only from 2 to 2^ mm. in length, 
their petioles do not exhibit any denned zone of small cells, 
destitute of chlorophyll and differing in shape exteriorly from 
the lower ones. Nevertheless, the cells at the place where a 
pulvinus will afterwards be developed are smaller (being on an 
average "015 mm. in length) than thoss in the lower parts of 
the same petiole, which gradually become larger in proceeding 
downwards, the largest being -030 mm. in length. At this ear]y 
age the cells of the blade are about *027 mm. in length. We 
thus see that the pulvinus is formed by the cells in the upper- 
most part of the petiole, continuing for only a short time to 
increase in length, then being arrested in their growth, accom- 
panied by the loss of their chlorophyll grains ; whilst the cells 
in the lower part of the petiole continue for a long time to 
increase in length, those of the epidermis becoming more conical. 
The singular fact of the cotyledons of this plant not sleeping at 
first is therefore due to the pulvinus not being developed at an 
early age. 

We learn from these two cases of Lotus and Oxalis, 
that the development of a pulvinus follows from the 
growth of the cells oyer a small defined space of the 
petiole being almost arrested at an early age. With 
Lotus Jacobseus the cells at first increase a little in 
length ; in Oxalis corniculata they decrease a little, 
owing to seli-division. A mass of such small cells 
forming a pulvinus, might therefore be either acquired 
or lost without any special difficulty, by different 
species in the same natural genus : and we know that 






Chap. II. DISTURBED PERIODIC MOVEMENTS. 123 

with seedlings of Trifoliurn, Lotus, and Oxalis some of 
the species have a well-developed pulvinus, and others 
have none, or one in a rudimentary condition. As the 
movements caused by the alternate turgescence of 
the cells in the two halves of a pulvinus, must be 
largely determined by the extensibility and subse- 
quent contraction of their walls, we can perhaps under- 
stand why a large number of small cells will be more 
efficient than a small number of large cells occupying 
the same space. As a pulvinus is formed by the 
arrestment of the growth of its cells, movements de- 
pendent on their action may be long-continued withou 
any increase in length of the part thus provided; 
and such long-continued movements seem to be one 
chief end gained by the development of a pulvinus. 
Long-continued movement would be impossible in any 
part, without an inordinate increase in its length, if the 
turgescence of the cells was always followed by growth. 
Disturbance of the Periodic Movements of Cotyledons by 
Light. — The hypocotyls and cotyledons of most seed- 
ling plants are, as is well known, extremely heliotropic ; 
but cotyledons, besides being heliotropic, are affected 
paratonically (to use Sachs' expression) by light ; that 
is, their daily periodic movements are greatly and 
quickly disturbed by changes in its intensity or by 
its absence. It is not that they cease to circumnutate 
in darkness, for in all the many cases observed by us 
they continued to do so; but the normal order of 
their movements in relation to the alternations of day 
and night is much disturbed or quite annulled. This 
holds good with species the cotyledons of which rise 
or sink so much at night that they may be said to 
sleep, as well as with others which rise only a little. 
But different species are affected in very different 
degrees by changes in the light. 



L24: DISTUEBED PEEIODIC MOVEMENTS. Chap. II. 

For instance, the cotyledons of Beta vulgaris, Solarium lycoper- 
sicum, Ctrinthe major, and Lvpinus luteus, when placed in dark- 
ness, moved down during the afternoon and early night, instead 
of rising as they would have done if they had been exposed tc 
the light. All the individuals of the Solanum did not behave 
in the same manner, for the cotyledons of one circumnutated 
about the same spot between 2.30 and 10 p.m. The cotyledons 
of a seedling of Oxalis corniculata, which was feebly illuminated 
from above, moved downwards during the first morning in the 
normal manner, but on the second morning it moved upwards. 
The cotyledons of Lotus Jacobceus were not affected by 4 h. of 
complete darkness, but when placed under a double skylight 
and thus feebly illuminated, they quite lost their periodical 
movements on the third morning. On the other hand, the 
cotyledons of Cucurbita ovifera moved in the normal manner 
during a whole day in darkness. 

Seedlings of Gitliago segetum were feebly illuminated from 
above in the morning before their cotyledons had expanded, and 
they remained closed for the next 40 h. Other seedlings were 
placed in the dark after their cotyledons had opened in the 
morning and these did not begin to close until about 4 h. had 
elapsed. The cotyledons of Oxalis rosra sank vertically down- 
wards after being left for 1 h. 20 m. in darkness ; but those of 
some other species of Oxalis were not affected by several hours 
of darkness. The cotyledons of several species of Cassia are 
eminently susceptible to changes in the degree of light to which 
they are exposed : thus seedlings of an unnamed S. Brazilian 
species (a large and beautiful tree) were brought out of the hot- 
house and placed on a table in the middle of a room with two 
north-east and one north-west window, so that they were fairly 
well illuminated, though of course less so than in the hot-house, 
the day being moderately bright ; and after 36 m. the cotyledons 
which had been horizontal rose up vertically and closed together 
as when asleep ; after thus remaining on the table for 1 h. 13 m. 
they began to open. The cotyledons of young seedlings of another 
Brazilian species and of C. neglecta, treated in the same manner, 
behaved similarly, excepting that they did not rise up quite so 
much : they again became horizontal after about an hour. 

Here is a more interesting case : seedlings of Cassia tora in 
two pots, which had stood for some time on the table in the 
room just described, had their cotyledons horizontal. One pot 
was now exposed for 2 h. to dull sunshine, and the cotyledons 



Ohap. it. sensitiy eness of cotyledons. 125 

remained horizontal ; it was then brought back to the table, and 
after 50 m. the cotyledons had risen 68° above the horizon. 
The other pot was placed during the same 2 h. behind a screen 
in the room, where the light was very obscure, and the cotyledons 
rose 63° above the horizon ; the pot was then replaced on the 
table, and after 50 m. the cotyledons had fallen 33°. These two 
pots with seedlings of the same age stood close together, and 
were exposed to exactly the same amount of light, yet the coty- 
ledons in the one pot were rising, whilst those in the other 
pot were at the same time sinking. This fact illustrates in a 
striking manner that their movements are not governed by the 
actual amount, but by a change in the intensity or degree of 
the light. A similar experiment was tried with two sets of seed- 
lings, both exposed to a dull light, but different in degree, and 
the result was the same. The movements of the cotyledons of this 
Cassia are, however, determined (as in many other cases) largely 
by habit or inheritance, independently of light; for seedlings 
which had been moderately illuminated during the day, were 
kept all night and on the following morning in complete dark- 
ness ; yet the cotyledons were partially open in the morning 
and remained open in the dark for about 6 h. The cotyledons 
in another pot, similarly treated on another occasion, were open 
at 7 a.m. and remained open in the dark for 4 h. 30 m., after 
which time they began to close. Yet these same seedlings, when 
brought in the middle of the day from a moderately bright 
into only a moderately dull light raised, as we have seen, their 
cotyledons high above the horizon. 

Sensitiveness of Cotyledons to contact. — This subject does not 
possess much interest, as it is not known that sensitiveness of this 
kind is of any service to seedling plants. We have observed cases 
in only four genera, though we have vainly observed the coty- 
ledons of many others. The genus Cassia seems to be pre-eminent 
in this respect : thus, the cotyledons of C. torn, when extended 
horizontally, were both lightly tapped with a very thin twig for 
3 m., and in the course of a few minutes they formed together 
an angle of 90°, so that each had risen 45°. A single cotyledon 
of another seedling was tapped in a like manner for 1 m., and it 
rose 27° in 9 m. ; and after eight additional minutes it had risen 
10° more ; the opposite cotyledon, which was not tapped, hardly 
moved at all. The cotyledons in all these cases became hori- 
zontal again in less than half an hour. The pulvinus is the most 
sensitive part, for on slightly pricking three cotyledons with a 



1.26 COTYLEDONS SENSITIVE Chap. IX 

pin in this part, they rose up vertically ; but the blade was found 
also to be sensitive, care having been taken that the pulvinus 
was not touched. Drops of water placed quietly on these coty- 
ledons produced no effect, but an extremely fine stream of water, 
ejected from a syringe, caused them to move upwards. When 
a pot of seedlings was rapidly hit with a stick and thus jarred, 
the cotyledons rose slightly. When a minute drop of nitric 
acid was placed on both pulvini of a seedling, the cotyledons 
rose so quickly that they could easily be seen to move, and 
almost immediately afterwards they began to fall; but the 
pulvini had been killed and became brown. 

The cotyledons of an unnamed species of Cassia (a large tree 
from S. Brazil) rose 31° in the course of 26 m. after the pulvini 
and the blades had both been rubbed during 1 m. with a twig ; 
but when the blade alone was similarly rubbed the cotyledons 
rose only 8°. The remarkably long and narrow cotyledons, of a 
third unnamed species from S. Brazil, did not move when their 
blades were rubbed on six occasions with a pointed stick for 
30 s. or for 1 m. ;'but when the pulvinus was rubbed and slightly 
pricked with a pin, the cotyledons rose in the course of a few 
minutes through an angle of 60°. Several cotyledons of 
C. neghda (likewise from S. Brazil) rose in from 5 m. to 15 m. to 
various angles between 16° and 34°, after being rubbed during 
1 m. with a twig. Their sensitiveness is retained to a somewhat 
advanced age, for the cotyledons of a little plant of 0. riegleda, 
34 days old and bearing three true leaves, rose when lightly 
pinched between the finger and thumb. -Some seedlings were 
exposed for 30 m. to a wind (temp. 50° F.) sufficiently strong to 
keep the cotyledons vibrating, but this to our surprise did not 
cause any movement. The cotyledons of four seedlings of the 
Indian ft gluuca were either rubbed with a thin twig for 2 m. or 
were lightly pinched : one rose 34° ; a second only 6° ; a third 
13°; and a fourth 17°. A cotyledon of C. floridu similarly 
treated rose 9° ; one of ft corymhosa rose 7h°, and one of the 
very distinct C. mimosoides only 6°. Those of C. pubescens did 
not appear to be in the least sensitive ; nor were those of ft 
ntdosa, but these latter are rather thick and fleshy, and do not 
rise at night or go to sleep. 

Smithia sen sit iva.— This plant belongs to a distinct sub-order of 
the Leguminosse from Cassia. Both cotyledons of an oldish 
seedling, with the first true leaf partially unfolded, were rubbed 
for 1 m. with a fine twig, and in 5 m. each rose 32°; thoj 



Chap. II. , TO CONTACT. 127 

remained in this position for 15 m., bnt when looked at again 
40 m. after the rubbing, each had fallen 14°. Both cotyledons of 
another and younger seedling were lightly rubbed in the same 
manner for 1 m., and after an interval of 32 m. each had risen 
30°. They were hardly at all sensitive to a fine jet of water. 
The cotyledons of S. Pfundii, an African water plant, are thick 
and fleshy ; they are not sensitive and do not go to sleep. 

Mimosa pudica and albida. — The blades of several cotyledons 
of both these plants were rubbed or slightly scratched with a 
needle during 1 m. or 2 m. ; but they did not move in the least. 
When, however, the pulvini of six cotyledons of M. pudica were 
thus scratched, two of them were slightly raised. In these two 
cases perhaps the pulvinus was accidentally pricked, for on 
pricking the pulvinus of another cotyledon it rose a little. It 
thus appears that the cotyledons of Mimosa are less sensitive 
than those of the previously mentioned plants.* 

Oxcilis sensitiva. — The blades and pulvini of two cotyledons, 
standing horizontally, were rubbed or rather tickled for 30 s. 
with a fine split bristle, and in 10 m. each had risen 48°; 
when looked at again in 35 m. after being rubbed they had 
risen 4° more ; after 30 additional minutes they were again hori- 
zontal. On hitting a pot rapidly with a stick for 1 m., the coty- 
ledons of two seedlings were considerably raised in the course 
of 11 m. A pot was carried a little distance on a tray and thus 
jolted ; and the cotyledons of four seedlings were all raised in 
10 m. ; after 17 m. one had risen 56°, a second 45°, a third almost 
90°, and a fourth 90°. After an additional interval of 40 m. three 
of them had re-expanded to a considerable extent. These obser- 
vations were made before we were aware at what an extraordi- 
narily rapid rate the cotyledons circumnutate, and are therefore 
liable to error. Nevertheless it is extremely improbable that the 
cotyledons in the eight cases given, should all have been rising 
at the time when they were irritated. The cotyledons of Oocalis 
Valdiviana and rosea were rubbed and did not exhibit any 
sensitiveness. 

Finally, there seems to exist some relation between 



* The sole notice which wo p. 865), " les cotyledons du M 

have met with on the sensitive- pudica tendent a se raprocher pal 

ness of cotyledons, relates to Mi- leurs faces supe'rienres lorsqu'oB 

mesa; for Au°\ P. De Candolle les ii'rite.'' 
says (' Phys. Ve'g.,' 1832, torn. ii. 



128 SENSITIVENESS OF COTYLEDONS. Chap IL 

the liabit of cotyledons rising vertically at night or 
going to sleep, and their sensitiveness, especially that 
of their pulvini, to a touch ; for all the above-named 
plants sleep at night. On the other hand, there are 
many plants the cotyledons of which sleep, and are 
not in the least sensitive. As the cotyledons of 
several species of Cassia are easily affected both by 
slightly diminished light and by contact, we thought 
that these two kinds of sensitiveness might be con- 
nected ; but this is not necessarily the case, for the 
cotyledons of Oxalis sensitiva did not rise when kept 
on one occasion for 1J h., and on a second occasion 
for nearly 4 h., in a dark closet. Some other coty- 
ledons, as those of Githago segetum, are much affected 
by a feeble light, but do not move when scratched by 
a needle. That with the same plant there is some 
relation between the sensitiveness of its cotyledons 
and leaves seems highly probable, for the above de- 
scribed Smithia and Oxalis have been called sensitiva, 
owing to their leaves being sensitive ; and though the 
leaves of the several species of Cassia are not sensitive 
to a touch, yet if a branch be shaken or syringed 
with water, they partially assume their nocturnal de- 
pendent position. But the relation between the sen- 
sitiveness to contact of the cotyledons and of the 
leaves of the same plant is not very close, as may be 
inferred from the cotyledons of Mimosa pudica being 
only slightly sensitive, whilst the leaves are well 
known to be so in the highest degree. Again, the 
leaves of Neptunia oleracea are very sensitive to a 
touch, whilst the cotyledons do not appear to be so iu 
any degree. 



Chap. III. SENSITIVENESS OF RADICLES. 120 






CHAPTEK 111. 

Sensitiveness of the Apex of the Radicle to Contact and to 
othek Irritants. 

Manner in which radicles bend when they encounter an obstacle in 
the soil — Vicia faba, tips of radicles highly sensitive to contact 
and other irritants — Effects of too high a temperature — Power of 
discriminating between objects attached on opposite sides — Tips of 
secondary radicles sensitive — Pisum, tips of radicles sensitive — 
Effects of such sensitiveness in overcoming geotropism— Secondary 
radicles — Phaseolus, tips of radicles hardly sensitive to contact 
but highly sensitive to caustic and to the removal of a slice — Tro- 
paoolum — Gossypiuin— Cucurbita — Eaphanus — iEsculus, tip not 
sensitive to slight contact, highly sensitive to caustic — Quercus, 
tip highly sensitive to contact — Power of discrimination — Zea 
tip highly sensitive, secondary radicles — Sensitiveness of radicles 
to moist air — Summary of chapter. 

In order to see how trie radicles of seedlings would 
pass over stones, roots, and other obstacles, which they 
must incessantly encounter in the soil, germinating 
beans (Vicia faba) were so placed that the tips of the 
radicles came into contact, almost rectangularly or 
at a high angle, with underlying plates of glass. In 
other cases the beans were turned about whilst their 
radicles were growing, so that they descended nearly 
vertically on their own smooth, almost flat, broad upper 
surfaces. The delicate root-cap, when it first touched 
any directly opposing surface, was a little flattened 
transversely ; the flattening soon became oblique, and 
in a few hours quite disappeared, the apex now point- 
ing at right angles, or at nearly right angles, to its 
former course. The radicle then seemed to glide in 
its new direction over the surface which had opposed 



130 SENSITIVENESS OF RADICLES. Chap. II L 

it, pressing on it with very little force. How. far such 
abrupt changes in its former course are aided by the 
circum nutation of the tip must be left doubtful. Thin 
slips of wood were cemented on more or less steeply 
inclined glass-plates, at right angles to the radicles 
which were gliding down them. Straight lines had 
been painted along the growing terminal part of some 
of these radicles, before they met the opposing slip 
of wood ; and the lines became sensibly curved in 2 h. 
after the apex had come into contact with the slips. 
In one case of a radicle, which was growing rather 
slowly, the root-cap, after encountering a rough slip 
of wood at right angles, was at first slightly flat- 
tened transversely : after an interval of 2 h. 30 m. 
the flattening became oblique ; and after an addi- 
tional 3 hours the flattening had wholly disappeared, 
and the apex now pointed at right angles to its former 
course. It then continued to grow in its new direc- 
tion alongside the slip of wood, until it came to the 
end of it, round which it bent rectangularly. Soon 
afterwards when coming 'to the edge of the plate of 
glass, it was again bent at a large angle, and de- 
scended perpendicularly into the damp sancl. 

When, as in the above cases, radicles encountered 
an obstacle at right angles to their course, the terminal 
growing part became curved for a length of between 
•3 and *4 of an inch (8-10 mm.), measured from the 
apex. This was well shown by the black lines which 
had been previously painted on them. The first and 
most obvious explanation of the curvature is, that it 
results merely from the mechanical resistance to the 
growth of the radicle in its original direction. Never- 
theless, this explanation did not seem to us satisfactory. 
The radicles did not present the appearance of having 
been subjected to a sufficient pressure to account for 



Chat. III. SENSITIVENESS OF RADICLES. 131 

tiieir curvature ; and Sachs has shown * that the 
growing part is more rigid than the part immediately 
above which has ceased to grow, so that the latter 
might have been expected to yield and become curved 
as soon as the apex encountered an unyielding object ; 
whereas it was the stiff growing part which became 
curved. Moreover, an object which yields with the 
greatest ease will deflect a radicle : thus, as we have 
seen, when the apex of the radicle of the beau 
encountered the polished surface of extremely thin 
tin-foil laid on soft sand, no impression was left on it 
yet the radicle became deflected at right angles. A 
second explanation occurred to us, namely, that even 
the gentlest pressure might check the growth of the 
apex, and in this case growth could continue only on 
one side, and thus the radicle would assume a rectan- 
gular form ; but this view leaves wholly unexplained 
the curvature of the upper part, extending for a length 
of 8-10 mm. 

We were therefore led to suspect that the apex 
was sensitive to contact, and that an effect was trans- 
mitted from it to the upper part of the radicle, which 
was thus excited to bend away from the touching object. 
As a little loop of fine thread hung on a tendril or 
on the petiole of a leaf-climbing plant, causes it to 
bend, we thought that any small hard object affixed 
to the tip of a radicle, freely suspended and growing 
in damp air, might cause it to bend, if it were sensitive, 
and yet would not offer any mechanical resistance to 
its growth. Full details will be given of the experi- 
ments which were tried, as the result proved remark- 
able. The fact of the apex of a radicle being sensitive 
to contact has never been observed, though, as we slial 1 



Arbeiten Bot. Inst. Wiirztmrg,' Heft iii. 1S73, p. 398. 



132 SENSITIVENESS OF THE APEX Chap. Ill 

hereafter see, Sacks discovered that the radicle a little 
above the apex is sensitive, and bends like a tendril 
towards the touching object. But when one side of the 
apex is pressed by any object, the growing part bends 
away from the object; and this seems a beautiful 
adaptation for avoiding obstacles in the soil, and, as 
we shall see, for following the lines of least resistance. 
Many organs, when touched, bend in one fixed direc- 
tion, such as the stamens of Berberis, the lobes of 
Dionsea, &c. ; and many organs, such as tendrils, whe- 
ther modified leaves or flower-peduncles, and some few 
stems, bend towards a touching object ; but no case, 
we believe, is known of an organ bending away from 
a touching object. 

Sensitiveness of the Apex of the Radicle of Vicia faba. 
— Common beans, after being soaked in water for 24 h., 
were pinned with the hilum downwards (in the manner 
followed by Sachs), inside the cork lids of glass-vessels, 
which were half filled with water ; the sides and the 
cork were well moistened, and light was excluded. 
As soon as the beans had protruded radicles, some to a 
length of less than a tenth of an inch, and others to 
a length of several tenths, little squares or oblongs of 
card were affixed to the short sloping sides of their 
conical tips. The squares therefore adhered obliquely 
with reference to the longitudinal axis of the radicle ; 
and this is a very necessary precaution, for if the bits 
of card accidentally became displaced, or were drawn 
by the viscid matter employed, so as to adhere parallel 
to the side of the radicle, although only a little way 
above the conical apex, the radicle did not bend in 
the peculiar manner which we are here considering. 
Squares of about the ^\jtk of an inch (i.e. about 1 J mm.), 
or oblong bits of nearly the same size, were found to 



Chap. III. OF THE RADICLE OF THE BEAN. 133 

be the most convenient and effective. We employed 
at first ordinary thin card, such as visiting cards, or 
bits of very thin glass, and various other objects ; but 
afterwards sand-paper was chiefly employed, for it was 
almost as stiff as thin card, and the roughened surface 
favoured its adhesion. At first we generally used very 
thick gum-water; and this of course, under the cir- 
cumstances, never dried in the least ; on the contrary, 
it sometimes seemed to absorb vapour, so that the bits 
of card became separated by a layer of fluid from the 
tip. When there was no such absorption and the card 
was not displaced, it acted well and caused the radicle 
to bend to the opposite side. I should state that 
thick gam-water by itself induces no action. In most 
cases the bits of card were touched with an extremely 
small quantity of a solution of shellac in spirits of 
wine, whieh had been left to evaporate until it was 
thick ; it then set hard in a few seconds, and fixed the 
bits of card well. When small drops of the shellac 
were placed on the tips without any card, they set into 
hard little beads, and these acted like any other hard 
object, causing the radicles to bend to the opposite 
side. Even extremely minute beads of the shellac 
occasionally acted in a slight degree, as will hereafter 
be described. But that it was the cards which chiefly 
acted in our many trials, was proved by coating one 
side of the tip with a little bit of goldbeaters' skin 
(which by itself hardly acts), and then fixing a bit of 
card to the skin with shellac which never came into 
contact with the radicle : nevertheless the radicle bent 
away from the attached card in the ordinary manner. 

Some preliminary trials were made, presently to 
be described, by which the proper temperature was 
determined, and then the following experiments were 
made. It should he piemised that the beans were 



134 



SENSITIVENESS OF THE APEX Chap. Ill, 



always fixed to the cork-lids, for the convenience of 
manipulation, with the edge from which the radicle 
and plumule protrudes, outwards ; and it must be 
remembered that owing to what we have called Sachs' 
curvature, the radicles, instead of growing perpendi- 
cularly downwards, often bend somewhat, even as much 




Vhia faba ; A, radicle beginning to bend from the attached little square 
of card ; B, bent at a rectangle ; C, bent into a circle or loop, with the 
tip beginning to bend downwards through the action of geotropism. 

as about 45° inwards, or under the suspended bean. 
Therefore when a square of card was fixed to the apex 
in front, the bowing induced by it coincided with Sachs' 
curvature, and could be distinguished from it only by 
being more strongly pronounced or by occurring more 
quickly. To avoid this source of doubt, the squares 



Chai\ III. OF THE EADICLE OF THE BEAN. 136 

wore fixed either behind, causing a curvature in direct 
opposition to that of Sachs', or more commonly to the 
right or left sides. For the sake of brevity, we will 
speak of the bits of card, &c, as fixed in front, or 
behind, or laterally. As the chief curvature of the 
radicle is at a little distance from the apex, and as 
the extreme terminal and basal portions are nearly 
straight, it is possible to estimate in a rough manner 
the amount of curvature by an angle ; and when it is 
said that the radicle became deflected at any angle 
from the perpendicular, this implies that the apex was 
turned upwards by so many degrees from the down- 
ward direction which it would naturally have followed, 
and to the side opposite to that to which the card was 
affixed. That the reader may have a clear idea of the 
kind of movement excited by the bits of attached 
card, we append here accurate sketches of three ger- 
minating beans thus treated, and selected out of 
several specimens to show the gradations in the 
degrees of curvature. We will now give in detail a 
series of experiments, and afterwards a summary of 
the results. 

In the first 12 trials, little squares or oblongs of sanded card, 
3/8 mm. in length, and 1*5 or only 0*9 mm. in breadth (i.e. "071 
ot an inch ia length and '059 or "035 of an inch in breadth) were 
fixed with shellac to the tips of the radicles. In the subsequent 
trials the little squares were only occasionally measured, but 
were of about the same size. 

(1.) A young radicle.. 4 mm. in length, had-a card fixed be- 
hind : after 9 h. deflected in the plane in which the bean is 
flattened, 50° from the perpendicular and from the card, and in 
opposition to Sachs' curvature : no change next morning, 23 h. 
from the time of attachment. 

(2.) Radicle 5'5 mm. in length, card fixed behind : after 9 h. 
deflected in the plane of the bean 20 c from the perpendicular 
and from the card, and in opposition to Sachs' curvature : after 
23 h. no change. 
10 



L36 SENSITIVENESS OF THE APEX Chap. Ill 

(3.) Eadicle 11 mm. in length, card fixed behind: after 9 h. 
deflected in the plane of the bean 40° from the perpendicular 
and from the card, and in opposition to Sachs' curvature. The 
tip of the radicle more curved than the upper part, but in the 
same plane. After 23 h. the extreme tip was slightly bent to- 
wards the card ; the general curvature of the radicle remaining 
the same. 

(4.) Eadicle 9 mm. long, card fixed behind and a little 
laterally: after i9h. deflected in the plane of the bean only 
about 7° or 8° from the perpendicular and from the card-, in 
opposition to Sachs' curvature. There was in addition a slight 
lateral curvature directed partly from the card. After 23 h. no 
change. 

(5.) Eadicle 8 mm. long, card affixed almost laterally : after 
9h. deflected 30° .from the perpendicular, in the plane of the 
bean and in opposition to Sachs' curvature ; also deflected in a 
plane at right angles to the above one, 20° from the perpen- 
dicular : after 23 h. no change. 

(6.) Eadicle 9 mm, long, card affixed in front : after 9 h. de- 
flected in the plane of the bean about 40° from the vertical, 
away from the card and in the direction of Sachs' curvature. 
Here therefore we have no evidence of the card being the 
cause of the deflection, except that a radicle never moves 
spontaneously, as far as we have seen, as much as 40° in the 
course of 9 h. After 23 h. no change. 

(7.) Eadicle 7 mm. long, card affixed to the back : after 9 h. 
the terminal part of the radicle deflected in the plane of the 
bean 20° from the vertical, away from the card and in opposition 
to Sachs' curvature. After 22 h. 30 m. this part of the radicle 
had become straight. 

(8.) Eadicle 12 mm. long, card affixed almost laterally : after 
9 h. deflected laterally in a plane at right angles to that of the 
bean between 40° and 50° from the vertical and from the card. 
In the plane of the bean itself the deflection amounted to 8° or 
9° from the vertical and from the card, in opposition to Sachs' 
curvature. After 22 h. 30 m. the extreme tip had become 
slightly curved towards the card. 

(9.) Card fixed laterally: after 11 h. 30m. no effect, the 
radicle being still almost vertical. 

(10.) Card fixed almost laterally: after 11 h. 30m. deflected 
90° from the vertical and from the card, in a plane inter- 
mediate between that of the bean itself and one at rigtt 






Chap. III. OF THE EADICLE OF THE BEAN. 187 

angles to it. Radicle consequently partially deflected from 
ISachs' curvature. 

(11.) Tip of jradicle protected with goldbeaters' skin, with a 
square of card of the usual dimensions affixed with shellac: 
after 11 h. greatly deflected in the plane of the bean, in the 
direction of Sachs' curvature, but to a much greater degree and 
in less time than ever occurs spontaneously. 

(12.) Tip of radicle protected as in last case : after 11 h. no 
effect, but after 24 h. 40 nf. radicle clearly deflected from the 
card. This slow action was probably due to a portion of the 
goldbeaters' skin having curled round and lightly touched, the 
opposite side of the tip and thus irritated it. 

(13.) A radicle of considerable length had a small square of 
card fixed with shellac to its apex laterally : after only 7 h. 15 m. 
a length of "4 of an inch from the apex, measured along the 
middle, was considerably curved from the side bearing the card. 

(14.) Case like the last in all respects, except that a length of 
only *25 of an inch of the radicle was thus deflected. 

(15.) A small square of card fixed with shellac to the apex of 
a young radicle; after 9 h. 15 m. deflected through 90° from the 
perpendicular and from the card. After 24 h. deflection much 
decreased, and after an additional day, reduced to 23° from the 
perpendicular. 

(10.) Square of card fixed with shellac behind the apex of a 
radicle, which from its position having been changed during 
growth had become very crooked; but the terminal portion 
was straight, and this became deflected to about 45° from 
the perpendicular and from the card, in opposition to Sachs' 
curvature. 

(17.) Square of card affixed with shellac : after 8 h. radicle 
curved at right angles from the perpendicular and from the 
card. After 15 additional hours curvature much decreased. 

(18.) Square of card affixed with shellac: after 8h. no effect; 
after 23 h. 3 m. from time of affixing, radicle much curved from 
the square. 

(19.) Square of card affixed with shellac : after 24 h. no effect, 
but the radicle had not grown well and seemed sickly. 

(20.) Square of card affixed with shellac : after 24 h. no effect. 

(21, 22.) Squares of card affixed with shellac : after 24 h. 
radicles of both curved at about 45° from the perpendicular and 
from the cards. 

(23.) Square of card fixed with shellac to young radicle : after 



138 SENSITIVENESS OF THE APEX Chap. Ill 

9k. very slightly curved from the card; after 24 h. tip curved 
towards card. Eefixed new square laterally, alter 9 h. distinctly 
curved from the card, and after 24 h. curved at right angles frcm 
the perpendicular and from the card. 

(24.) A rather large oblong piece of card fixed with shellac to 
apex ; after 24 h. no effect, but the card was found not to be 
touching the apex. A small square was now refixed with 
shellac ; after 16 h. slight deflection from the perpendicular 
and from the card. After an additional day the radicle became 
almost straight. 

(25.) Square of card fixed laterally to apex of young radicle; 
after 9 h. deflection from the perpendicular considerable ; after 
24 h. deflection reduced. Eefixed a fresh square with shellac: 
after 24 h. deflection about 40° from the perpendicular and from 
the card. 

(26.) A very small square of card fixed with shellac to apex of 
young radicle : after 9 h. the deflection from the perpendicular 
'and from the card amounted to nearly a right angle; after 24 h. 
deflection much reduced ; after an additional 24 h. radicle almost 
straight. 

(27.) Square of card fixed with shellac to apex of young 
radicle : after 9 h. deflection from the card and from the perpen- 
dicular a right angle ; next morning quite straight. Refixed 
a square laterally with shellac ; after 9 h. a little deflection, 
which after 24 h. increased to nearly 20° from the perpendicular 
and from the card. 

(23.) Square of card fixed with shellac; after 9 h. some 
deflection; next morning the card dropped off; refixed it with 
shellac ; it again became loose and was refixed ; and now on the 
third trial the radicle was deflected after 14 h. at right a ogles 
from the card. 

(29.) A small square of card was first fixed with thick gum- 
water to the apex. It produced a slight effect but soon fell 
off. A similar square was now affixed laterally with shellac : 
after 9 h. the radicle was deflected nearly 45° from the perpen- 
dicular and from the card. Alter 36 additional hours angle of 
deflection reduced to about 30°. 

(30.) A very small piece, less than -^th of an inch square, of 
thin tin-foil fixed with shellac to the apex of a young radicle ; 
after 24 h. no effect. Tin-foil removed, and a small square of 
sanded card fixed with shellac ; after 9 \\ deflection at nearly 
right angles from the perpendicular and from the card. Next 



Chap. III. OF THE RADIC1 E OF THE BEAN. 139 

morning deflection reduced to about 40° from the perpen- 
dicular. 

(31.) A splinter of thin glass gummed to apex, after 9 h. no 
effect, but it was then found not to be touching the apex of the 
radicle. Next morning a square of card was fixed with shellac 
to it, and after 9 h. radicle greatly deflected from the card. 
After two additional days the deflection had decreased and was 
only 35° from the perpendicular. 

(32.) Small square of sanded card, attached with thick gum- 
water laterally to the apex of a long straight radicle: after 9 h. 
greatly deflected from the perpendicular and from the card. 
Curvature extended for a length of *22 of an inch from the 
apex. After 3 additional hours terminal portion deflected at 
right angles from the perpendicular. Next morning the curved 
portion was '36 in length. 

(33.) Square of card gummed to apex : after 15 h. deflected at 
nearly 90° from the perpendicular and from the card. 

(34.) Small oblong of sanded card gummed to apex: after 
15 h. deflected 90° from the perpendicular and from the card : 
in the course of the three following days the terminal portion 
became much contorted and ultimately coiled into a helix. 

(35.) Square of card gummed to apex: after 9 h. deflected from 
card: after 24 h. from time of attachment greatly deflected 
obliquely and partly in opposition to Sachs' curvature. 

(36.) Small piece of card, rather less than ^th of an inch 
square, gummed to apex : in 9 h. considerably deflected from 
card and in opposition to Sachs' curvature ; after 24 h. greatly 
deflected in the same direction. After an additional day the 
extreme tip was curved towards the card. 

(37.) Square of card, gummed to apex in front, caused after 
8 h. 30 m. hardly any effect ; refixed fresh square laterally, after 
15 h. deflected almost 90° from the perpendicular and from the 
card. After 2 additional days deflection much reduced. 

(38.) Square of card gummed to apex : after 9 h. much deflec- 
tion, which after 24 h. from time of fixing increased to nearly 
90°. After an additional day terminal portion was curled into 
a loop, and on the following day into a helix. 

(39.) Small oblong piece of card gummed to apex, nearly in 
front, but a little to one side; in 9 h. slightly deflected in the 
direction of Sachs' curvature, but rather obliquely, and to 
side opposite to card. Next day more curved in the same 
direction, and after 2 additional days ceiled into a ring. 



110 SENSITIVENESS OF THE APEX Chap. III. 

(10.) Square of card gummed to apex: after 9 h. slightly 
curved from card ; uext morning radicle straight, and apex had 
grown beyond the card. Eefixed another square laterally with 
shellac ; in 9 h. deflected laterally, but also in the direction of 
Sachs' curvature. After 2 additional days' curvature consider- 
ably increased in the same direction. 

(41.) Little square of tin-foil fixed with gum to one side of 
apex of a young and short radicle : after 15 h. no effect, but 
tin-foil had become displaced. A little square of card was now 
gummed to one side of apex, which after 8 h. 40 m. was slightly 
deflected; in 21 h. from the time of attachment deflected at 90° 
from the perpendicular and from the card ; after 9 additional 
hours became hooked, with the apex pointing to the zenith. In 
3 days from the time of attachment the terminal portion of the 
radicle formed a ring or circle. 

(42.) A little square of thick letter-paper gummed to the 
apex of a radicle, which after 9 h. was deflected from it. In 
24 h. from time when the paper was affixed the deflection much 
increased, and after 2 additional days it amounted to 50° from 
the perpendicular and from the paper. 

(43.) A narrow chip of a quill was fixed with shellac to the 
apex of a radicle. After 9 h. no effect; after 24 h. moderate 
deflection, but now the quill had ceased to touch the apex. 
Removed quill and gummed a little square of card to apex, 
which after 8 h. caused slight deflection. On the fourth day 
from the first attachment of any object, the extreme tip was 
curved towards the card. 

(44.) A rather long and narrow splinter of extremely thin 
glass, fixed with shellac to apex, it caused in 9 h. slight 
deflection, which disappeared in 24 h. ; the splinter was then 
found not touching the apex. It was twice refixed, with nearly 
similar results, that is, it caused slight deflection, which soon 
disappeared. On the fourth day from the time of first attach- 
ment the tip was bent towards the splinter. 

From these experiments it is clear that the apex of 
the radicle of the bean is sensitive to contact, and 
that it causes the upper part to bend away from the 
touching object. But before giving a summary of the 
results, it will be convenient briefly to give a few othei 
observations. Bits of very thin glass and little square? 



Chap. III. OF THE RADICLE OF THE BEA3. 141 

of common card were affixed with thick gum- water to 
the tips of the radicles of seven beans, as a pre- 
liminary trial. Six of these were plainly acted on, 
and in two cases the radicles became coiled up into 
complete loops. One radicle was curved into a semi- 
circle in so short a period as 6 h. 10 m. The 
seventh radicle which was not affected was apparently 
sickly, as it became brown on the following day ; so 
that it formed no real exception. Some of these trials 
were made in the early spring during cold weather in 
a sitting-room, and others in a greenhouse, but the 
temperature was not recorded. These six striking 
cases almost convinced us. that the apex was sensitive, 
but of course we determined to make many more trials. 
As we had noticed that the radicles grew much more 
quickly when subjected to considerable heat, and as 
we imagined that heat would increase their sensitive- 
ness, vessels with germinating beans suspended in 
damp air were placed on a chimney-piece, where they 
were subjected during the greater part of the day to a 
temperature of between 69° and 72° F. ; some, how- 
ever, were placed in the hot-house where the tempera- 
ture was rather higher. Above two dozen beans were 
thus tried ; and when a square of glass or card did 
not act, it was removed, and a fresh one affixed, this 
beins; often done thrice to the same radicle. There- 
fore between five and six dozen trials were altogether 
made. But there was moderately distinct deflection 
from the perpendicular and from the attached object 
in only one radicle out of this large number of cases. 
In five other cases there was very slight and doubtful 
deflection. We were astonished at this result, and 
concluded that we had made some inexplicable mis- 
take in the first six experiments. But before finally 
relinquishing the subject, we resolved to make one 



142 SENSITIVENESS OF THE APEX Chap. III. 

other trial, for it occurred to us that sensitiveness is 
easily affected by external conditions, and that radicles 
growing naturally in the earth in the early spring 
would not be subjected to a temperature nearly so 
high as 70° F. We therefore allowed the radicles 
of 12 beans to grow at a temperature of between 
55° and 60° F. The result was that in every one of 
these cases (included in the above-described experi- 
ments) the radicle was deflected in the course of a few 
hours from the attached object. All the above re- 
corded successful trials, and some others presently to 
be given, were made in a sitting-room at the tempera- 
tures just specified. It therefore appears that a tem- 
perature of about, or rather above, 70° F. destroys 
the sensitiveness of the radicles, either directly, or 
indirectly through abnormally accelerated growth ; 
and this curious fact probably explains why Sachs, 
who expressly states that his beans were kept at a 
high temperature, failed to detect the sensitiveness of 
the apex of the radicle. 

But other causes interfere with this sensibility. 
Eighteen radicles were tried with little squares of 
sanded card, some affixed with shellac and some with 
gum- water, during the few last days of 1878, and few 
first days of the next year. They were kept in a room 
at the proper temperature during the day, but were 
probably too cold at night, as there was a hard frost at 
the time. The radicles looked healthy but grew very 
slowly. The result was that only 6 out of the 18 
were deflected from the attached cards, and this only 
to a slight degree and at a very slow rate. T«hese 
radicles therefore presented a striking contrast with 
the 41 above described. On March 6th and 7th, when 
the temperature of the room varied between 53° and 
59° F., eleven germinating beans were tried in the 



Ohap. [II OF THE RADICLE OF' THE BEAN. 143 

same manner, and now every one of the radicles 
became curved away from the cards, though one was 
only slightly deflected. Some horticulturists believe 
that certain kinds of seeds will not germinate pro- 
perly in the middle of the winter, although kept at a 
right temperature. If there really is any proper period 
for the germination of the bean, the feeble degree of 
sensibility of the above radicles may have resulted 
from the trial having been made in the middle of the 
winter, and not simply from the nights being too cold. 
Lastly, the radicles of four beans, which from some 
innate cause germinated later than all the others of 
the same lot, and which grew slowly though appearing 
healthy, were similarly tried, and even after 24 h. they 
were hardly at all deflected from the attached cards 
We may therefore infer that any cause which renders 
the growth of the radicles either slower or more rapid 
than the normal rate, lessens or annuls the sensibility 
of their tips to contact. It deserves particular atten- 
tion that when the attached objects failed to act, there 
was no bending of any kind, excepting Sachs' curva- 
ture. The force of our evidence would have been 
greatly weakened if occasionally, though rarely, the 
radicles had become curved in any direction inde- 
pendently of the attached objects. In the foregoing 
numbered paragraphs, however, it may be observed 
that the extreme tip sometimes becomes, after a con- 
siderable interval of time, abruptly curved towards the 
bit of card ; but this is a totally distinct phenomenon; 
as will presently be explained. 

Summary of the Results of the foregoing Experiments 
on the Radicles of Vicia faba. — Altogether little squares 
(about ^th of an inch), generally of sanded paper 
as stiff as thin card (between *15 and -20 mm. in 
thickness), sometimes of ordinary card, or little frag- 



L44 SENSITIVENESS OF THE APEX Chap. Ill 

ments of very thin glass, &c, were affixed at different 
times to one side of the conical tips of 55 radicles. 
The 11 last-mentioned cases, but not the preliminary 
ones, are here included. The squares, &c, were most 
commonly affixed with shellac, but in 19 cases with 
thick gum-water. When the latter was used, the 
squares were sometimes found, as previously stated, 
to be separated from the apex by a layer of thick 
fluid, so that there was no contact, and conse- 
quently no bending of the radicle ; and such few 
cases were not recorded. But in every instance in 
which shellac was employed, unless the square fell 
off very soon, the result was recorded. In several 
instances when the squares became displaced, so as 
to stand parallel to the radicle, or were separated by 
fluid from the apex, or soon fell off, fresh squares 
were attached, and these cases (described under the 
numbered paragraphs) are here included. Out of 
55 radicles experimented on under the proper tempe- 
rature, 52 became bent, generally to a considerable 
extent from the perpendicular, and away from the 
side to which the object was attached. Of the three 
failures, one can be accounted for, as the radicle 
became sickly on the following day; and a second 
was observed only during 11 h. 30 m. As in several 
cases the terminal growing part of the radicle continued 
for some time to bend from the attached object, it 
formed itself into a hook, with the apex pointing to 
the zenith, or even into a ring, and occasionally into a 
spire or helix. It is remarkable that these latter cases 
occurred more frequently when objects were attached 
with thick gum-water, which never became dry, than 
when shellac was employed. The curvature was often 
well-marked in from 7 h. to 11 h. ; and in one instance 
a semicircle was formed in 6 h. 10 m. from the time 



Chap. III. OF THE KADIOLE OF THE BEAN. Il5 

of attachment. But in order to see the phenomenon 
as well displayed as in the above described cases, it is 
indispensable that the bits of card, &c, should be 
made to adhere closely to one side of the conical 
apex ; that healthy radicles should be selected and 
kept at not too high or too low a temperature, and 
apparently that the trials should not be made in the 
middle of the winter. 

In ten instances, radicles which had curved away 
from a square of card or other object attached to their 
tips, straightened themselves to a certain extent, or 
even completely, in the course of from one to two days 
from the time of attachment. This was more espe- 
cially apt to occur when the curvature was slight. 
But in one instance (No. 27) a radicle which in 9 h. 
had been deflected about 90° from the perpendicular, 
became quite straight in 24 h. from the period of 
attachment. With No. 26, the radicle was almost 
straight in 48 h. We at first attributed the straighten- 
ing process to the radicles becoming accustomed to a 
slight stimulus, in the same manner as a tendril or 
sensitive petiole becomes accustomed to a very light 
loop of thread, and unbends itself though the loop 
remains still suspended ; but Sachs states * that radicles 
of the bean placed horizontally in damp air after 
curving downwards through geotropism, straighten 
themselves a little by growth along their lower or 
concave sides. Why this should occur is not clear ; 
but perhaps it likewise occurred in the above ten 
cases. There is another occasional movement which 
must not be passed over : the tip of the radicle, for a 
length of from 2 to 3 mm., was found in six instances, 



Arbeiten Bot. Instit., Wiirzburg,' Heft iii. p. 456. 



140 SENSITIVENESS OF THE APEX Chap, lit 

after an interval of about 24 or more hours, bent 
towards the bit of still attached card, — that is, in a 
direction exactly opposite to the previously induced 
curvature of the whole growing part for a length of 
from 7 to 8 mm. This occurred chiefly when the first 
curvature was small, and when an object had been 
affixed more than once to the apex of the same radicle. 
The attachment of a bit of card by shellac to one 
side of the tender apex may sometimes mechanically 
prevent its growth ; or the application of thick gum- 
water more than once to the same side may injure it ; 
and then checked growth on this side with continued 
growth on the opposite and unaffected side would 
account for the reversed curvature of the apex. 

Various trials were made for ascertaining, as far 
as we could, the nature and degree of irritation to 
which the apex must be subjected, in order that the 
terminal growing part should bend away, as if to 
avoid the cause of irritation. We have seen in the 
numbered experiments, that a little square of rather 
thick letter-paper gummed to the apex induced, 
though slowly, considerable deflection. Judging from 
several cases in which various objects had been affixed 
with gum, and had soon become separated from the 
apex by a layer of fluid, as well as from some trials 
in which drops of thick gum-water alone had been 
applied, this fluid never causes bending. We have 
also seen in the numbered experiments that narrow 
splinters of quill and of very thin glass, affixed with 
shellac, caused only a slight degree of deflection, and 
this may perhaps have been clue to the shellac 
itself. Little squares of goldbeaters' skin, which is 
excessively thin, were damped, and thus made to 
adhere to one side of the tips of two radicles ; one of 
these, after 24 h., produced no effect ; nor did the 



Ohap. III. OF THE RADICLE OF THE BEAN. L47 

other in 8 h., within which time squares of card usually 
act ; but after 24 h. there was slight deflection. 

An oval bead, or rather cake, of dried shellac, 
1 • 01 mm. in length and * 63 in breadth, caused a 
radicle to become deflected at nearly right angles in 
the course of only 6 h. ; but after 23 h. it had nearly 
straightened itself. A very small quantity of dissolved 
shellac was spread over a bit of card, and tbe tips of 
9 radicles were touched laterally with it ; only two of 
them became slightly deflected to the side opposite 
to that bearing the speck of dried shellac, and they 
afterwards straightened themselves. These specks 
were removed, and both together weighed less than 
T ^th of a grain; so that a weight of rather Jess 
than 2-^o th of a grain (0"32 mgs.) sufficed to excite 
movement in two out of the nine radicles. Here 
then we have apparently reached nearly the minimum 
weight which will act. 

A moderately thick bristle (which on measurement 
was found rather flattened, being 0*33 mm. in one 
diameter, and 0"20 mm. in the other) was cut into 
lengths of about ^th. of an inch. These after being 
touched with thick gum-water, were placed on the tip 
of eleven radicles. Three of them were affected ; one 
being deflected in 8 h. 15 m. to an angle of about 90° 
from the perpendicular ; a second to the same amount 
when looked at after 9 h. ; but after 24 h. from the 
time of first attachment the deflection had decreased 
to only 19° ; the third was only slightly deflected 
after 9 h., and the bit of bristle was then found not 
touching the apex ; it was replaced, and after 15 
additional hours the deflection amounted to 26° from 
the perpendicular. The remaining eight radicles 
were not at all acted on by the bits of bristle, so that 
we here appear to have nearly reached the minimum 



148 SENSITIVENESS OF THE APEX Chap. III. 

of size of an object which will act on the radicle of 
the bean. But it is remarkable that when the bits of 
bristle did act, that they should have acted so quickly 
and efficiently. 

As the apex of a radicle in penetrating the ground 
must be pressed on all sides, we wished to learn 
whether it could distinguish between harder or more 
resisting, and softer substances. A square of the sanded 
paper, almost as stiff as card, and a square of extremely 
thin paper (too thin for writing on), of exactly the 
same size (about ^V^h °f an lncn )> were fixed with 
shellac on opposite sides of the apices of 12 suspended 
radicles. The sanded card was between 0*15 and 
0-20 mm. (or between 0-0059 and 0-0079 of an inch), 
and the thin paper only 0*045 mm. (or 0-00176 of an 
inch) in thickness. In 8 out of the 12 cases there 
could be no doubt that the radicle was deflected from 
the side to which the card-like paper was attached, and 
towards the opposite side, bearing the very thin paper. 
This occurred in some instances in 9 h., but in others 
not until 24 h. had elapsed. Moreover, some of the 
four failures can hardly be considered as really failures : 
thus, in one of them, in which the radicle remained 
quite straight, the square of thin paper was found, 
when both were removed from the apex, to have been 
so thickly coated with shellac that it was almost as 
stiff as the card : in the second case, the radicle was 
bent upwards into a semicircle, but the deflection 
was not directly from the side bearing the card, and 
this was explained by the two squares having become 
cemented laterally together, forming a sort of stiff 
gable, from which the radicle was deflected : in the 
third case, the square of card had been fixed by 
mistake in front, and though there was deflection 
from it, this might have been due to Sachs' curvature , 



Chap. III. OF THE RADICLE OF THE BEAN. 149 

in the fourth case alone no reason could be assigned 
why the radicle had not been at all deflected. These 
experiments suffice to prove that the apex of the 
radicle possesses the extraordinary power of discri- 
minating between thin card and very thin paper, and 
is deflected from the side pressed by the more re- 
sisting or harder substance. 

Some trials were next made by irritating the tips 
without any object being left in contact with them. 
Nine radicles, suspended over water, had their tips 
rubbed, each six times with a needle, with sufficient 
force to shake the whole bean ; the temperature was 
favourable, viz. about 63° F. In 7 out of these cases 
no effect whatever was produced ; in the eighth case 
the radicle became slightly deflected from, and in thp 
ninth case slightly deflected towards, the rubbed side : 
but these two latter opposed curvatures were probably 
accidental, as radicles do not always grow perfectly 
straight downwards. The tips of two other radicles 
were rubbed in the same manner for 15 seconds with 
a little round twig, two others for 30 seconds, and two 
others for 1 minute, but without any effect being pro- 
duced. We may therefore conclude from these 15 
trials that the radicles are not sensitive to temporary 
contact, but are acted on only by prolonged, though 
very slight, pressure. 

We then tried the effects of cutting off a very thin 
slice parallel to one of the sloping sides of the apex 5 
as we thought that the wound would cause prolonged 
irritation, which might induce bending towards the 
opposite side, as in the case of an attached object. 
Two preliminary trials were made : firstly, slices were 
cut from the radicles of 6 beans suspended in damp 
air, with a pair of scissors, which, though sharp, 
probably caused considerable crushing, and no curva- 



[50 SENSITIVENESS OF THE APEX Chap 1HT. 

ture followed. Secondly, thin slices were cut with a 
razor obliquely off the tips of three radicles similarly 
suspended ; and after 44 h. two were found plainly 
bent from the sliced surface ; and the third, the whole 
apex of w T hich had been cut off obliquely by accident, 
was curled upwards over the bean, but it was not 
clearly ascertained whether the curvature had been at 
first directed from the cut surface. These results led 
us to pursue the experiment, and 18 radicles, w 7 hich 
had grown vertically downwards in damp air, had one 
side of their conical tips sliced off with a razor. The 
tips were allowed just to enter the water in the jars, 
and they were exposed to a temperature 14°-16° C. 
(57°-61° F.). The observations were made at dif- 
ferent times. Three were examined 12 h. after beimr 
sliced, and were all slightly curved from the cut 
surface ; and the curvature increased considerably after 
an additional 12 h. Eight were examined after 19 h. : 
four after 22 h. 30 m. ; and three after 25 h. The 
final result was that out of the 18 radicles thus tried, 
13 were plainly bent from the cut surface after the 
above intervals of time ; and one other became so 
after an additional interval of 13 h. 30 m. So that 
only 4 out of the 18 radicles were not acted on. To 
these 18 cases the 3 previously mentioned ones should 
be added. It may, therefore, be concluded that a thin 
slice removed by a razor from one side of the conical 
apex of the radicle causes irritation, like that from an 
attached object, and induces curvature from the injured 
surface. 

Lastly, dry caustic (nitrate of silver) was employer! 
to irritate one side of the apex. If one side of the 
apex or of the whole terminal growing part of a 
radicle, is by any means killed or badly injured, the 
other side continues to grow ; and this causes the pan 



Chap. III. OF THE RADICLE OF THE BEAN. 151 

to bend over towards the injured side.* But i: the 
following experiments we endeavoured, generally with 
success, to irritate the tips on one side, without ladly 
injuring thern. This was effected by first drying the 
tip as far as possible with blotting-paper, though it still 
remained somewhat damp, and then touching it once 
with quite dry caustic. Seventeen radicles were thus 
treated, and were suspended in moist air over water at 
a temperature of 58 J F. They were examined after 
an interval of 21 h. or 24 h. The tips of two were 
found blackened equally all round, so that they could 
tell nothing and were rejected, 15 being ]eft. Of 
these, 10 were curved from the side which had been 
touched, where there was a minute brown or blackish 
mark. Five of these radicles, three of which were 
already slightly deflected, were allowed to enter the 
water in the jar, and were re-examined after an addi- 
tional interval of 27 h. (i.e. in 48 h. after the appli- 
cation of the caustic), and now four of them had 
become hooked, being bent from the discoloured side 
with their points directed to the zenith ; the fifth 
remained unaffected and straight. Thus 11 radicles 
out of the 15 were acted on. But the curvature of 
the four just described was so plain, that they alone 
would have sufficed to show that the radicles of the 
bean bend away from that side of the apex which has 
been slightly irritated by caustic. 

TJie power of an Irritant on the apex of the Radicle 



* Ciesielski found this to be the pended over water, with a thick 

case (' Untersuchungen iiber die layer of grease, which is very 

Abwartskrummung der Wurzel,' injurious or even fatal to grow- 

1871, p. 28) after burning with ing parts; for after 48 hours 

heated platinum one side of a five of these radicles were curved 

radicle. So did we when we towards the greased side, two 

painted longitudinally half of the remaining straight. 
* T hole length of 7 radicles, sus- 

11 



L52 SENSITIVENESS OF THE APEX Chap. Ill 

of the Bean, compared ivith tJiat of Geotrojrism. — We 
know that when a little square of card or other 
object is fixed to one side of the tip of a vertically 
dependent radicle, the growing part bends from it 
often into a semicircle, in opposition to geotropism, 
which force is conquered by the effect of the irri- 
tation from the attached object. Radicles were there- 
fore extended horizontally in damp air, kept at 
the proper low temperature for full sensitiveness, 
and squares of card were affixed with shellac on the 
loiver sides of their tips, so that if the squares 
acted, the terminal growing part would curve upwards. 
Firstly, eight beans were so placed that their short, 
young, horizontally extended radicles would be simul- 
taneously acted on both by geotropism and by Sachs' 
curvature, if the latter came into play ; and they all 
eight became bowed downwards to the centre of the 
earth in 20 h., excepting one which was only slightly 
acted on. Two of them were a little bowed downwards 
in only 5 h. ! Therefore the cards, affixed to the lower 
sides of their tips, seemed to produce no effect ; and 
geotropism easily conquered the effects of the irritation 
thus caused. Secondly, 5 oldish radicles, 1J- inch in 
length, and therefore less sensitive than the above- 
mentioned young ones, were similarly placed and 
similarly treated. From what has been seen on many 
other occasions, it may be safely inferred that if they 
had been suspended vertically they would have bent 
away from the cards ; and if they had been extended 
horizontally, without cards attached to them, they 
would have quickly bent vertically downwards through 
geotropism ; but the result was that two of these 
radicles were still horizontal after 23 h. ; two were 
curved only slightly, and the fifth as much as 40° 
beneath the horizon. Thirdly, 5 beans were fastened 



Chap. J II. OF THE EADICLE OF THE BEAN. 153 

with their flat surfaces parallel to the cork-lid, so that 
Sachs' curvature would not tend to make the hori- 
zontally extended radicles turn either upwards or 
downwards, and little squares of card were affixed as 
before, to the lower sides of their tips. The result 
was that all five radicles were bent down, or towards 
the centre of the earth, after only 8 h. 20 m. At 
the same time and within the same jars, 3 radicles of 
the same age, with squares affixed to one side, were 
suspended vertically ; and after 8 h. 20 m. they were 
considerably deflected from the cards, and therefore 
curved upwards in opposition to geotropism. In these 
latter cases the irritation from the squares had over- 
powered geotropism ; whilst in the former cases, in 
which the radicles were extended horizontally, geo- 
tropism had overpowered the irritation Thus within 
the same jars, some of the radicles were curving 
upw r ards and others downwards at the same time— 
these opposite movements depending on whether the 
radicles, when the squares were first attached to them, 
projected vertically down, or were extended horizon- 
tally. This difference in their behaviour seems at first 
inexplicable, but can, we believe, be simply explained 
by the difference between the initial power of the two 
forces under the above circumstances, combined with 
the well-known principle of the after-effects of a sti- 
mulus. When a vouno- and sensitive radicle is extended 
horizontally, with a square attached to the lower side 
of the tip, geotropism acts on it at right angles, and, 
as we have seen, is then evidently more efficient than 
the irritation from the square ; and the power of geo- 
tropism will be strengthened at each successive period 
by its previous action — that is, by its after-effects. 
On the other hand, when a square is affixed to a 
rertically dependent radicle, and the apex begins to 



1 54 SENSITIVENESS OF THE EADICLE. Chap. Ill 

curve upwards, this movement will be opposed by geo- 
tropism acting only at a very oblique angle, and the 
irritation from the card will be strengthened by its 
previous action. We may therefore conclude that the 
initial power of an irritant on the apex of the radicle 
of the bean, is less than that of geotropism when 
acting at right angles, but greater than that of geo- 
tropism when acting obliquely on it. 

Sensitiveness of the tips of the Secondary Radicles of the 
Bsan to contact. — All the previous observations relate 
to the main or primary radicle. Some beans suspended 
to cork-lids, with their radicles dipping into water, had 
developed secondary or lateral radicles, wilich were 
afterwards kept in very damp air, at the proper low 
temperature for full sensitiveness. They projected, 
as usual, almost horizontally, with only a slight 
downward curvature, and retained this position 
during several davs. Sachs has shown* that these 
secondary roots are acted on in a peculiar manner by 
geotropism, so that if displaced they reassume their 
former sub -horizontal position, and do not bend verti- 
cally downwards like the primary radicle. Minute 
squares of the stiff sanded paper were affixed by 
means of shellac (but in some instances with thick 
gum-water) to the tips of 39 secondary radicles of 
different ages, generally the uppermost ones. Most 
of the squares were fixed to the lower sides of the apex, 
so that if they acted the radicle would bend upwards ; 
but some were fixed laterally, and a few on the upper 
side. Owing to the extreme tenuity of these radicles, 
it was very difficult to attach the square to the 
actual apex. Whether owing to this or some other 
circumstance, only nine of the squares induced an y 



Arbeitec Eot. Inst., Wiirzburg,' Heft iv. 1874, p. 605-617. 



Chap. Ill SENSITIVENESS OF THE EADICLE. 155 

curvature. The curvature amounted in some cases to 
about 45° above the horizon, in others to 90°, and then 
the tip pointed to the zenith. In one instance a 
distinct upward curvature was observed in 8 h. 15 m., 
but usually not until 24 h. had elapsed. Although 
only 9 out of 39 radicles were affected, yet the curva- 
ture was so distinct in several of them, that there could 
be no doubt that the tip is sensitive to slight contact, 
and that the growing part bends away from the touch- 
ing object. It is possible that some secondary radicles 
are more sensitive than others ; for Sachs has proved * 
the interesting fact that each individual secondary 
radicle possesses its own peculiar constitution. 

Sensitiveness to contact of the Primary Radicle, a little 
above the apex, in the Bean ( Vicia faba) and Pea (Pisum 
sativum). — The sensitiveness of the apex of the radicle 
in the previously described cases, and the consequent 
curvature of the upper part from the touching object 
or other source of irritation, is the more remarkable, 
because Sachs | has shown that pressure at the distance 
of a few millimeters above the apex causes the radicle 
to bend, like a tendril, towards the touching object. 
By fixing pins so that they pressed against the radicles 
of beans suspended vertically in damp air, we saw this 
kind of curvature ; but rubbing the part with a twig 
or needle for a few minutes produced no effect. Haber- 
landt remarks,:]: that these radicles in breaking through 
the seed-coats often rub and press against the ruptured 
edges, and consequently bend round them. As little 
squares of the card-like paper affixed with shellac to 
the tips were highly efficient in causing the radicles 
to bend away from them, similar pieces (of about -^th 



* 'Arbeileti Bot. Instit., Wiirz- \ 'DieScliutzeinrichtungendej 

burg,' Heft. iv. 1874, p. 620. Keimpflauze,' 1877, p. 25. 

t Ibid. Heft iii. 1873, p. 437. 



)56 SENSITIVENESS OF THE Chap. Ill 

inch square, or rather less) were attached in the same 
manner to one side of the radicle at a distance of 3 or 
4 mm. above the apex. In our first trial on 15 radicles 
no effect was produced. In a second trial on the same 
number, three became abruptly curved (but only one 
strongly) towards the card within 24 h. From these 
cases we may infer that the pressure from a bit of card 
affixed with shellac to one side above the apex, is hardly 
a sufficient irritant ; but that it occasionally causes the 
radicle to bend like a tendril towards this side. 

We next tried the effect of rubbing several radicles 
at a distance of 4 mm. from the apex for a few seconds 
with lunar caustic (nitrate of silver) ; and although the 
radicles had been wiped dry and the stick of caustic 
was dry, yet the part rubbed was much injured and a 
slight permanent depression was left. In such cases 
the opposite side continues to grow, and the radicle 
necessarily becomes bent towards the injured side. 
But when a point 4 mm. from the apex was momen- 
tarily touched with dry caustic, it was only faintly 
discoloured, and no permanent injury was caused. This- 
was shown by several radicles thus treated straighten- 
ing themselves after one or two days ; yet at first they 
became curved towards the touched side, as if they had 
been there subjected to slight continued pressure. 
These cases deserve notice, because when one side of 
the apex was just touched with caustic, the radicle, as 
we have seen, curved itself in an opposite direction, that 
is, away from the touched side. 

The radicle of the common pea at a point a little 
above the apex is rather more sensitive to continued 
pressure than that of the bean, and bends towards the 
pressed side.* We experimented on a variety (York- 



Saclis, ' A beiten Bot. Institut, Wiirzburg,' Heft iii. p. 438. 



CHAr. III. UPPER PART OF THE RADICLE. 15? 

sJiire Hero) which has a much wrinkled tough skin, 
too large for the included cotyledons ; so that out of 
30 peas which had been soaked for 24 h. and allowed 
to germinate on damp sand, the radicles of three were 
unable to escape, and were crumpled up in a strange 
manner within the skin ; four other radicles were 
abruptly bent round the edges of the ruptured skin 
against which they had pressed. Such abnormalities 
would probably never, or very rarely, occur with forms 
developed in a state of nature and subjected to natural 
selection. One of the four radicles just mentioned in 
doubling backwards came into contact with the pin 
by which the pea was fixed to the cork-lid ; and now it 
bent at right angles round the pin, in a direction quite 
different from that of the first curvature due to contact 
with the ruptured skin ; and it thus afforded a good 
illustration of the tendril-like sensitiveness of the 
radicle a little above the apex. 

Little squares of the card-like paper were next 
affixed to radicles of the pea at 4 mm. above the apex, 
in the same manner as with the bean. Twenty-eight 
radicles suspended vertically over water were thus 
treated on different occasions, and 13 of them became 
curved towards the cards. The greatest degree of 
curvature amounted to 62° from the perpendicular; 
but so large an angle was only once formed. On one 
occasion a slight curvature was perceptible after 5 h. 
45 m., and it was generally well-marked after 14 h. 
There can therefore be no doubt that with the pea, 
irritation from a bit of card attached to one side of the 
radicle above the apex suffices to induce curvature. 

Squares of card were attached to one side of the tips 
of 11 radicles within the same jars in which the above 
trials were made, and five of them became plainly, 
and one slightly, curved away from this side. Other 



158 SENSITIVENESS OF THE APEX Chap. Ill 

analogous cases will be immediately described. The 
fact is here mentioned because it was a striking spec- 
tacle, showing the difference in the sensitiveness of 
the radicle in different parts, to behold in the same 
jar one set of radicles curved away from the squares ou 
their tips, and another set curved towards the squares 
attached a little higher up. Moreover, the kind of 
curvature in the two cases is different. The squares 
attached above the apex cause the radicle to bend 
abruptly, the part above and beneath remaining nearly 
straight ; so that here there is little or no transmitted 
effect. On the other hand, the squares attached to 
the apex affect the radicle for a length of from about 
4 to even 8 mm., inducing in most cases a sym- 
metrical curvature ; so that here some influence is 
transmitted from the apex for this distance along the 
radicle. 

Pisum sativum (var. Yorkshire Hero) : Sensitiveness of 
the apex of the Radicle. — Little squares of the same card- 
like paper were affixed (April 24th) with shellac to 
one side of the apex of 10 vertically suspended radicles : 
the temperature of the water in the bottom of the jars 
was 60°-61° F. Most of these radicles were acted on 
in 8 h. 30 m. ; and eight of them became in the course 
of 24 h. conspicuously, and the remaining two slightly, 
deflected from the perpendicular and from the side 
bearing the attached squares. Thus all were acted on ; 
but it will suffice to describe two conspicuous cases. 
Tn one the terminal portion of the radicle was bent at 
right angles (A, Fig. 66) after 24 h. ; and in the other 
(B) it had by this time become hooked, with the apex 
pointing to the zenith. The two bits of card here used 
were *07 inch in length and *04 inch in breadth. Two 
other radicles, which after 8 h. 30 m. were moderately 
deflected, became straight again after 24 h. Another 



Chap. III. 



OF THE EADICLE OF THE PEA. 



159 



trial was made in the same manner with 15 radicles ; 
but from circumstances, not worth explaining, they 
were only once and briefly examined after the short 

Fig. 66. 





B. 

Pisum sativum ; deflection produced within 24 hours in the growth of 
vertically dependent radicles, by little squares of card affixed with 
shellac to one side of apex : A, bent at light angles ; B, hooked. 

interval of 5 h. 30 m. ; and we merely record in our 
notes " almost all bent slightly from the perpendicular, 
and away from the squares ; the deflection amounting 
in one or two instances to nearly a rectangle." These 
two sets of cases, especially the first one, prove that 
the apex of the radicle is sensitive to slight contact 
and that the upper part bends from the touching 
object. Nevertheless, on June 1st and 4th, 8 other 
radicles were tried in the same manner at a tempera- 
ture of 58°-60° F., and after 24 h. only 1 was decidedly 
bent from the card, 4 slightly, 2 doubtfully, and 1 not 
in the least. The amount of curvature was unaccount- 
ably small ; but all the radicles which were at all bent, 
were bent away from the cards. 

We now tried the effects of widely different tempera- 
tures on the sensitiveness of these radicles with squares 



160 SENSITIVENESS OF THE APEX Chap. Ill 

of card attached to their tips. Firstly, 13 peas, most 
of them having very short and young radicles, were 
placed in an ice-box, in which the temperature rose 
during three days from 44° to 47° F. They grew slowly, 
but 10 out of the 13 became in the course of the three 
days very slightly curved from the squares ; the other 
3 were not affected ; so that this temperature was too 
low for any high degree of sensitiveness or for much 
movement. Jars with 13 other radicles were next 
placed on a chimney-piece, where they were subjected 
to a temperature of between 68° and 72° F., and 
after 24 n., 4 were conspicuously curved from the 
cards, 2 slightly, and 7 not at all ; so that this tem- 
perature was rather too high. Lastly, 12 radicles 
were subjected to a temperature varying between 
72° and 85° F., and none of them were in the least 
affected by the squares. The above several, trials, 
especially the first recorded one, indicate that the 
most favourable temperature for the sensitiveness of 
the radicle of the pea is about 60° F. 

The tips of 6 vertically dependent radicles were 
touched once with dry caustic, in the manner described 
under Vicia faba. After 24 h. four of them were bent 
from the side bearing a minute black mark ; and the 
curvature increased in one case after 38 h., and in 
another case after 48 h., until the terminal part pro- 
jected almost horizontally. The two remaining ra- 
dicles were not affected. 

With radicles of the bean, when extended horizontally 
in damp air, geotropism always conquered the effects 
of the irritation caused by squares of card attached to 
the lower sides of their tips. A similar experiment 
was tried on 13 radicles of the pea ; the squares being 
attached with shellac, and the temperature between 
58°~60° F. The result was somewhat different ; for 



Chap. III. 



OF THE KADICLE OF THE PEA. 



161 



these radicles are either less strongly acted on by 
geotropism, or, what is more probable, are more sen- 
sitive to contact. After a time geotropism always 
prevailed, but its action was often delayed ; and in 
three instances there was a most curious struggle 
between geotropism and the irritation caused by the 
cards. Eour of the 13 radicles were a little curved 
downwards within 6 or 8 h., always reckoning from 
the time when the squares were first attached, and 
after 23 h. three of them pointed vertically down- 
wards, and the fourth at an angle of 45° beneath the 
horizon. These four radicles therefore did not seem 



Fig. 67. 





Pisum sativum: a radicle extended horizontally in damp air with a little 
square of card affixed to the lower side of its tip, causing it to bend 
upwards in opposition to geotropism. The deflection of the radicle 
after 21 hours is shown at A, and of the same radicle after 45 hours at 
B, now forming a loop. 

to have been at all affected by the attached squares. 
Four others were not acted on by geotropism within 
the first 6 or 8 h., but after 23 h. were much bowed 
down. Two others remained almost horizontal for 
23 h., but afterwards were acted on. So that in these 
latter six cases the action of geotropism was much 
delayed. The eleventh radicle was slightly curved 
down after 8 h., but when looked at again after 23 h. 
the terminal portion was curved upwards ; if it had 



lf>2 SENSITIVENESS OF THE APEX Chap. III. 

been longer observed, the tip no doubt would have 
been found again curved down, and it would have 
formed a loop as in the following case. The twelfth 
radicle after 6 h. was slightly curved downwards ; but 
when looked at again after 21 h., this curvature had 
disappeared and the apex pointed upwards ; after 30 h. 
the radicle formed a hook, as shown at A (Fig. 67) ; 
which hook after 45 h. was converted into a loop (B). 
The thirteenth radicle after 6 h. was slightly curved 
downwards, but within 21 h. had curved considerably 
up, and then down again at an angle of 45° beneath 
the horizon, afterwards becoming perpendicular. In 
these three last cases geotropism and the irritation 
caused by the attached squares alternately prevailed 
in a highly remarkable manner ; geotropism being 
ultimately victorious. 

Similar experiments were not always quite so suc- 
cessful as in the above cases. Thus 6 radicles, horizon- 
tally extended with attached squares, were tried on 
June 8th at a proper temperature, and after 7 h. 30 m. 
none were in the least curved upwards and none were 
distinctly geotropic ; whereas of 6 radicles without any 
attached squares, which served as standards of com- 
parison or controls, 3 became slightly and 3 almost 
rectangularly geotropic within the 7 h. 30 m. ; but 
after 23 h. the two lots were equally geotropic. On 
July 10th another trial was made with 6 horizontally 
extended radicles, with squares attached in the same 
manner beneath their tips ; and after 7 h. 30 m., 4 were 
slightly geotropic, 1 remained horizontal, and 1 was 
curved upwards in opposition to gravity or geotropism. 
This latter radicle after 48 h. formed a loop, like that 
at B (Fig. 67). 

An analogous trial was now made, but instead of 
attaching squares of card to the lower sides of the 



Chap. in. OF THE KADICLE OF PIIASEOLUS. 16o 

tips, these were touched with dry caustic. The details 
of the experiment will be given -in the chapter on 
Geotropism, and it will suffice here to say that 10 
peas, with radicles extended horizontally and not cau- 
terised, were laid on and under damp friable peat ; 
these, which served as standards or controls, as well as 
10 others which had been touched on the upper side 
with the caustic, all became strongly geotropic in 24 h. 
Nine radicles, similarly placed, had their tips touched 
on the lower side with the caustic ; and after 24 h., 

3 were slightly geotropic, 2 remained horizontal, and 

4 were bowed upwards in opposition to gravity and to 
geotropism. This upward curvature was distinctly 
visible in 8 h. 45 m. after the lower sides of the tips 
had been cauterised. 

Little squares of card were affixed with shellac on 
two occasions to the tips of 22 young and short 
secondary radicles, which had been emitted from the 
primary radicle whilst growing in water, but were now 
suspended in damp air. Besides the difficulty of 
attaching the squares to such finely pointed objects 
as were these radicles, the temperature was too high, 
— varying on the first occasion from 72° to 77° F., and 
on the second being almost steadily 78° F. ; and this 
probably lessened the sensitiveness of the tips. The 
result was that after an interval of 8 h. 30 m,, 6 of the 

122 radicles were bowed upwards (one of them greatly) 
in opposition to gravity, and 2 laterally ; the remain- 
ing 14 were not affected. Considering the unfavour- 
able circumstances, and bearing in mind the case of 
the bean,, the evidence appears sufficient to show that 
the tips of the secondary radicles of the pea are 
sensitive to slight contact. 
Phaseolus multiflorus : Sensitiveness of the apex of the 



161 SENSITIVENESS OF THE APEX Chap. Ill 

of various sizes of the same card-like paper, also with 
bits of thin glass and rough cinders, affixed with shellac 
to one side of the apex. Eather large drops of the 
dissolved shellac were also placed on them and allowed 
to set into hard beads. The specimens were subjected 
to various temperatures between 60° and 72° F., more 
commonly at about the latter. But out of this con- 
siderable number of trials only 5 radicles were plainly 
bent, and 8 others slightly or even doubtfully, from 
the attached objects ; the remaining 46 not being at 
all affected. It is therefore clear that the tips of the 
radicles of this Phaseolus are much less sensitive to 
contact than are those of the bean or pea. We 
thought that they might be sensitive to harder 
pressure, but after several trials we could not devise 
any method for pressing harder on one side of the 
apex than on the other, without at the same time 
offering mechanical resistance to its growth. We 
therefore tried other irritants. 

The tips of 13 radicles, dried with blotting-paper, 
were thrice touched or just rubbed on one side 
with dry nitrate of silver. They were rubbed thrice, 
because we supposed from the foregoing trials, that 
the tips were not highly sensitive. After 24 h. the 
tips were found greatly blackened ; 6 were blackened 
equally all round, so that no curvature to any one 
side could be expected ; 6 were much blackened on 
one side for a length of about T ' th of an inch, and 
this length became curved at right angles towards the 
blackened surface, the curvature afterwards increasing 
in several instances until little hooks were formed. 
] t was manifest that the blackened side was so much 
injured that it could not grow, whilst the opposite 
side continued to grow. One alone out of these 13 
radicles became curved from the blackened side, the 



Chap. in. OF THE RADICLE OF PHASEOLUS. 16S 

curvature extending for some little distance above 
the apex. 

After the experience thus gained, the tips of six 
almost dry radicles were once touched with the dry 
caustic on one side ; and after an interval of 10 m. 
were allowed to enter water, which was kept at a 
temperature of 65°-67° F. The result was that after 
an interval of 8 h. a minute blackish speck could 
just be distinguished on one side of the apex of five 
of these radicles, all of which became curved towards 
the opposite side — in two cases at about an angle 
of 45° — in two other cases at nearly a rectangle — and 
in the fifth case at above a rectangle, so that the apex 
was a little hooked ; in this latter case the black mark 
was rather larger than in the others. After 24 h. 
from the application of the caustic, the curvature of 
three of these radicles (including the hooked one) had 
diminished ; in the fourth it remained the same, and 
in the fifth it had increased, the tip being now hooked. 
It has been said that after 8 h. black specks could 
be seen on one side of the apex of five of the six 
radicles ; on the sixth the speck, which was extremely 
minute, was on the actual apex and therefore central ; 
and this radicle alone did not become curved. It was 
therefore again touched on one side with caustic, and 
after 15 h. 30 m. was found curved from the perpen- 
dicular and from the blackened side at an angle of 34°, 
which increased in nine additional hours to 54°. 

It is therefore certain that the apex of the radicle 
of this Phaseolus is extremely sensitive to caustic, 
more so than that of the bean, though the latter is 
far more sensitive to pressure. In the experiments 
just given, the curvature from the slightly cauterised 
side of the tip, extended along the radicle for a 
length of nearly 10 mm. ; whereas in the first set 



166 SENSITIVENESS OF THE APEX Chap. Ill 

of experiments, when the tips of several were greatly 
blackened and injured on one side, so that their growth 
was arrested, a length of less than 3 mm. became 
curved towards the much blackened side, owing to the 
continued growth of the opposite side. This differ- 
ence in the results is interesting, for it shows that too 
strong an irritant does not induce any transmitted 
effect, and does not cause the adjoining, upper and 
growing part of the radicle to bend. We have analo- 
gous cases with Drosera, for a strong solution of car- 
bonate of ammonia when absorbed by the glands, or 
too great heat suddenly applied to them, or crushing 
them, does not cause the basal part of the tentacles 
to bend, whilst a weak solution of the carbonate, or a 
moderate heat, or slight pressure always induces such 
bending. Similar results were observed with Dionsea 
and Pinguicula. 

The effect of cutting off with a razor a thin slice 
from one side of the conical apex of 14 young and 
short radicles was next tried. Six of them after beiug 
operated on were suspended in damp air ; the tips of 
the other eight, similarly suspended, were allowed to 
enter water at a temperature of about 65° F. It was 
recorded^ in each case which side of the apex had 
been sliced off, and when they were afterwards 
examined the direction of the curvature was noted, 
before the record was consulted. Of the six radicles 
in damp air, three had their tips curved after an 
interval of 10 h. 15 m. directly away from the sliced 
surface, whilst the other three were not affected and 
remained straight ; nevertheless, one of them after 
13 additional hours became slightly curved from the 
sliced surface. Of the eight radicles with their tips 
immersed in water, seven were plainly curved away 
from the sliced surfaces after 10 h. 15 m. ; and with 



CkAP. III. OF THE KADICLE OF TKOP^EOLUM. 167 

respect to the eighth which remained quite straight, 
too thick a slice had been accidentally removed, so 
that it hardly formed a real exception to the general 
result. When the seven radicles were looked at 
again, after an interval of 23 h. from the time of 
slicing, two had become distorted ; four were deflected 
at an angle of about 70° from the perpendicular and 
from the cut surface ; and one was deflected at nearly 
90°, so that it projected almost horizontally, but with 
the extreme tip now beginning to bend downwards 
through the action of geotropism. It is therefore 
manifest that a thin slice cut off one side of the conical 
apex, causes the upper growing part of the radicle of 
this Phaseolus to bend, through the transmitted effects 
of the irritation, away from the sliced surface. 

Tropseolum majus : Sensitiveness of the apex of the 
Radicle to contact. — Little squares of card were attached 
with shellac to one side of the tips of 19 radicles, some 
of which were subjected to 78 J F., and others to a 
much lower temperature. Only 3 became plainly 
curved from the squares, 5 slightly, 4 doubtfully, 
and 7 not at all. These seeds were, as we believed, 
old, so we procured a fresh lot, and now the results 
were widely different. Twenty-three were tried in 
the same manner ; five of the squares produced no 
effect, but three of these cases were no real exceptions, 
for in two of them the squares had slipped and were 
parallel to the apex, and in the third the shellac was 
in excess and had spread equally all round the apex. 
One radicle was deflected only slightly from the 
perpendicular and from the card ; whilst seventeen 
were plainly deflected. The angles in several of these 
latter cases varied between 40° and 65° from the 
perpendicular ; and in two of them it amounted after 
15 h. or 16 h. to about 90°. In one instance a loop 
12 



168 SENSITIVENESS OF THE APEX Chap. Ill 

was nearly completed in 16 h. There can, therefore, 
be no doubt that the apex is higl.ly sensitive to slight 
contact, and that the upper part of the radicle bends 
away from the touching object. 

Gossijpium herbaceum : Sensitiveness of the apex of the 
Badicle. — Radicles were experimented on in the same 
manner as before, but they proved ill-fitted for our 
purpose, as they soon became unhealthy when sus- 
pended in damp air. Of 38 radicles thus suspended, 
at temperatures varying from 66° to 69° F., with 
squares of card attached to their tips, 9 were plainly 
and 7 slightly or even doubtfully deflected from the 
squares and from the perpendicular; 22 not being 
affected. We thought that perhaps the above tempera- 
ture was not high enough, so 19 radicles with attached 
squares, likewise suspended in damp air, were subjected 
to a temperature of from 74° to 79° F., but not one of 
them was acted on, and they soon became unhealthy. 
Lastly, 19 radicles were suspended in water at a tem- 
perature from 70° to 75° F., with bits of glass or 
squares of the card attached to their tips by means of 
Canada-balsam or asphalte, which adhered rather better 
than shellac beneath the water. The radicles did not 
keep healthy for long. The result was that 6 were 
plainly and 2 doubtfully deflected from the attached 
objects and the perpendicular; 11 not being affected. 
The evidence consequently is hardly conclusive, 
though from the two sets of cases tried under a 
moderate temperature, it is probable that the radicles 
are sensitive to contact ; and would be more so under 
favourable conditions. 

Fifteen radicles which had germinated in friable peat 
were suspended vertically over water. Seven of them 
served as controls, and they remained quite straight 
during 24 h. The tips of the other eight radicles 



Ohap. III. OF THE KADICLE OF CUCUKBITA. 169 

were just touched with dry caustic on one side. Aftei 
only 5 h. 10 m. five of thern were slightly curved 
from the perpendicular and from the side beariug the 
little blackish marks. After 8 h. 40 m.. 4 out of 
these 5 were deflected at angles between 15° and 65° 
from the perpendicular. On the other hand, one 
which had been slightly curved after 5 h. 10 in., now 
became straight. After 24 h. the curvature in two 
cases had considerably increased ; also in four other 
cases, but these latter radicles had now become so 
contorted, some being turned upwards, that it could no 
longer be ascertained whether they were still curved 
from the cauterised side. The control specimens ex- 
hibited no such irregular growth, and the two sets 
presented a striking contrast. Out of the 8 radicles 
which had been touched with caustic, two alone were 
not affected, and the marks left on their tips by the 
caustic were extremely minute. These marks in all 
cases were oval or elongated ; they were measured in 
three instances, and found to be of nearly the same 
size, viz. § of a mm. in length. Bearing this fact in 
mind, it should be observed that the length of the 
curved part of the radicle, which had become deflected 
from the cauterised side in the course of 8 h. 40 m., 
was found to be in three cases 6, 7, and 9 mm. 

Cucurbit a ovifera : Sensitiveness of the cqyex of the Ba~ 
dicle. — The tips proved ill-fitted for the attachment of 
cards, as they ars extremely fine and flexible. More- 
over, owing to the hypocotyls being soon developed 
and becoming arched, the whole radicle is quickly 
displaced and confusion is thus caused. A large 
number of trials were made, but without any definite 
result, excepting on tw^o occasions, when out of 23 
radicles 10 were deflected from the attached squares 



170 SENSITIVENESS OF THE APEX Chap. Ill 

of card, and 13 were not acted on. Bather large 
Bquares, though difficult to affix, seemed more efficient 
than very small ones. 

We were much more successful with caustic ; but in 
our first trial, 15 radicles were too much cauterised, 
and only two became curved from the blackened side ; 
the others being either killed on one side, or blackened 
equally all round. In our next trial the dried tips 
of 11 radicles were touched momentarily with dry 
caustic, and after a few minutes were immersed in 
water. The elongated marks thus caused were never 
black, only brown, and about \ mm. in length, or 
even less. In 4 h. 30 m. after the cauterisation, 6 of 
them were plainly curved from the side with the 
brown mark, 4 slightly, and 1 not at all. The latter 
proved unhealthy, and never grew ; and the marks on 
2 of the 4 slightly curved radicles were excessively 
minute, one being distinguishable only with the aid 
of a lens. Of 10 control specimens tried in the same 
jars at the same time, not one was in the least curved. 
In 8 h. 40 m. after the cauterisation, 5 of the radicles 
out of the 10 (the one unhealthy one being omitted) 
were deflected at about 90°, and 3 at about 45° from 
the perpendicular and from the side bearing the 
brown mark. After 24 h. all 10 radicles had in- 
creased immensely in length ; in 5 of them the curva- 
ture was nearly the same, in 2 it had increased, and 
in 3 it had decreased. The contrast presented by the 
10 controls, after both the 8 h. 40 m. and the 24 h. 
intervals, was very great ; for. they had continued to 
grow vertically downwards, excepting two which, from 
some unknown cause, had become somewhat tortuous. 

In the chapter on Geotropism we shall see that 
10 radicles of this plant were extended horizontally on 
and beneath damp friable peat, under which conditions 



Chap. III. OF THE RADICLE OF RAPHANUS. 171 

they grow better and more naturally than in damp 
air; and their tips were slightly cauterised on the 
lower side, brown marks about J mm. in length 
being thus caused. Uncauterised specimens similarly 
placed became much bent downwards through geo- 
tropism in the course of 5 or 6 hours. After 8 h. 
only 3 of the cauterised ones were bowed downwards, 
and this in a slight degree ; 4 remained horizontal ; 
and 3 were curved upwards in opposition to geo- 
tropism and from the side bearing the brown mark. 
Ten other specimens had their tips cauterised at the 
same time and in the same degree, on the upper 
side ; and this, if it produced any effect, would tend 
to increase the power of geotropism ; and all these 
radicles were strongly bowed downwards after 8 h. 
From the several foregoing facts, there can be no 
doubt that the cauterisation of the tip of the radicle 
of this Cucurbita on one side, if done lightly enough, 
causes the whole growing part to bend to the opposite 
side. 

Baphanus sativus : Sensitiveness of the apex of tlie 
Radicle. — We here encountered many difficulties in 
our trials, both with squares of card and with caustic ; 
for when seeds were pinned to a cork-lid, many of the 
radicles, to which nothing had been done, grew irre- 
gularly, often curving upwards, as if attracted by the 
damp surface above ; and when they were immersed 
in water they likewise often grew irregularly. We 
did not therefore dare to trust our experiments with 
attached squares of card ; nevertheless some of them 
seemed to indicate that the tips were sensitive to 
contact. Our trials with caustic generally failed from 
the difficulty of not injuring too greatly the extremely 
fine tips. Out of 7 radicles thus tried, one became 
bowed after 22 h. at an angle of 60°, a second at 40° 



172 SENSITIVENESS OF THE APEX Chap. Ill 

and a third very slightly from the perpendicular and 
from the cauterised side. 

Mseulus hippoeastanum : Sensitiveness of the apex oj 
the Radicle. — Bits of glass and squares of card were 
affixed with shellac or gum-water to the tips of 12 
radicles of the horse-chestnut ; and when these objects 
fell off, they were reflxed ; but not in a single instance 
was any curvature thus caused. These massive 
radicles, one of which was above 2 inches in length 
and *3 inch in diameter at its base, seemed insensible 
to so slight a stimulus as any small attached object. 
Nevertheless, when the apex encountered an obstacle 
in its downward course, the growing part became sc 
uniformly and symmetrically curved, that its appear- 
ance indicated not mere mechanical bending, but 
increased growth along the whole convex side, due to 
the irritation of the apex. 

That this is the correct view may be inferred from 
the effects of the more powerful stimulus of caustic. 
The bending from the cauterised side occurred much 
slower than in the previously described species, and it 
will perhaps be worth while to give our trials in 
detail. 

The seeds germinated in sawdust, and one side of the tips of 
the radicles were slightly rubbed once with dry nitrate of silver; 
and after a few minutes were allowed to dip into water. They 
were subjected to a rather varying temperature, generally 
between 52° and 58° F. A few cases have not been thought 
worth recording, in which the whole tip was blackened, or in 
which the seedling soon became unhealthy. 

(1.) The radicle was slightly deflected from the cauterised 
side in one day (i.e. 24 fa.) ; in three days it stood at 60° from 
the xjerpendicular ; in four days at 90° ; on the fifth day it was 
curved up about 40° above the horizon ; so that it had passed 
through an angle of 130° in the five days, and this was the 
greatest amount of curvature observed. 

(2.) In two days radicle slightly deflected ; after seven days 



Chap. III. OF THE KADICLE OF .ESCULUS. 173 

deflected 69° from the perpendicular and from the cauterised 
side; after eight days the angle amounted to nearly 90°. 

(3.) After one day slight deflection, but the cauterised mark 
was so faint that the same side was again touched with caustic. 
In four days from the first touch deflection amounted to 7b°, 
which in an additional day increased to 90°. 

(4.) After two days slight deflection, which during the next 
three days certainly increased but never became great ; the 
radicle did not grow well and died on the eighth day. 

(5.) After two days very slight deflection ; but this on the 
fourth day amounted to 56° from the perpendicular and from 
the cauterised side. 

(6.) After three days doubtfully, but after four days certainly 
deflected from the cauterised side. On the fifth day deflection 
amounted to 45° from the perpendicular, and this on the seventh 
day increased to about 90°. 

(7.) After two days slightly deflected ; on the third day the 
deflection amounted to 25° from the perpendicular, and this 
did not afterwards increase. 

(8.) After one day deflection distinct; on the third day i* 
amounted to 44°, and on the fourth day to 72° from the perpen- 
dicular and the cauterised side. 

(9.) After two days deflection slight, yet distinct ; on the 
third day the tip was again touched on the same side with 
caustic and thus killed. 

(10.) After one day slight deflection, which after six days 
increased to 50° from the perpendicular and the cauterised side. 

(11.) After one day decided deflection, which after six days 
increased to 62° from the perpendicular and from the cauterised 
side. 

(12.) After one clay slight deflection, which on the second day 
amounted to 35°, on the fourth day to 50°, and the sixth day 
to 63° from the perpendicular and the cauterised side. 

(13.) Whole tip blackened, but more on one side than the 
other ; on the fourth day slightly, and on the sixth day greatly 
deflected from the more blackened side ; the deflection on the 
rinth day amounted to 90° from the perpendicular. 

(14.) Whole tip blackened in the same manner as in the last 
case ; on the second day decided deflection from the more 
blackened side, which increased on the seventh day to nearlj 
90° ; on the following day the radicle appeared unhealthy. 

^15 ) Here we had the anomalous case of a radicle bending 



174: SENSITIVENESS OF THE APEX Chap. Ul 

slightly towards the cauterised side on the first day, and con- 
tinuing to do so for the next three days, when the deflection 
amounted to about 90° from the perpendicular. The cause 
appeared to lie in the tendril-like sensitiveness of the upper part 
of the radicle, against which the point of a large triangular flap 
of the seed-coats pressed with considerable force ; and this 
irritation apparently conquered that from the cauterised apex. 

These several cases show beyond doubt that the 
irritation of one side of the apex, excites the upper 
part of the radicle to bend slowly towards the opposite 
side. This fact was well exhibited in one lot of five 
seeds pinned to the cork-lid of a jar ; for when after 
6 days the lid was turned upside down and viewed 
from directly above, the little black marks made by the 
caustic were now all distinctly visible on the upper 
sides of the tips of the laterally bowed radicles. 

A thin slice was shaved off with a razor from one 
side of the tips of 22 radicles, in the manner described 
under the common bean ; but this kind of irritation 
did not prove very effective. Only 7 out of the 22 
radicles became moderately deflected in from 3 to 5 
days from the sliced surface, and several of the others 
grew irregularly. The evidence, therefore, is far from 
conclusive. 

Quercus robur: Sensitiveness of the apex of the Radicle. 
— The tips of the radicles of the common oak are fully 
as sensitive to slight contact as are those of any plant 
examined by us. They remained healthy in damp air 
for 10 days, but grew slowly. Squares of the card- 
like paper were fixed with shellac to the tips of 15 
radicles, and ten of these became conspicuously bowed 
from the perpendicular and from the squares ; two 
slightly, and three not at all. But two of the latter 
were not real exceptions, as they were at first very 
short, and hardly grew afterwards. Some of the more 



L'dap. III. OF THE EADICLE OF QUEKCUS. 



175 



remarkable cases are worth describing. The radicles 
were examined on each successive morning, at nearly 
the same hour, that is, after intervals of 24 h. 



Fig. 68. 




No. 1. This radicle suffered from a series of accidents, and 
acted in an anomalous manner, for the apex appeared at first 
insensible and afterwards sensitive to contact. The first square 
was attached on Oct. 19th ; on the 21st the 
radicle was not at all curved, and the square 
was accidentally knocked off; it was refixed 
on the 22nd. and the radicle became slightly 
carved from the square, but tne curvature 
disappeared on the 23rd, when the square 
was removed and refixed. No curvature en- 
sued, and the square was again accidentally 
knocked off, and refixed. On the morning of 
the 27th it was washed off by having reached 
the water in the bottom of the jar. The 
square was refixed, and on the 29th, that 
is, ten days alter the first square had been 
attached, and two days after the attachment 
of the last square, the radicle had grown to 
the great length of 3 2 inches, and now 
the terminal growing part had become bent 
away from the square into a hook (see 
Fig. 68). 

No. 2. Square attached on the 19th ; on 
the 20th radicle slightly deflected from it 
and from the perpendicular; on the 21st 
deflected at nearly right angles ; it remained during the next 
two days in this position, but on the 25th the upward curva- 
ture was lessened through the action of geotropism, and still 
more so on the 26th. 

No. 8. Square attached on the 19th; on the 21st a trace of 
curvature from the square, which amounted on the 22nd to 
about 40°, and on the 23rd to 53° from the perpendicular. 

No. 4. Square attached on the 21st; on the 22nd trace of 
curvature from the square ; on the 23rd completely hooked 
with the point turned up to the zenith. Three days afterwards 
(i.e. 26th) the curvature had wholly disappeared and the apex 
pointed perpendicularly downwards. 

No. 5. Square attached on the 21st ; on the 22nd decided 



}uercus robur : radicle 
with square of card 
attached to one side 
of apex, causing it 
to become hooked. 
Drawing one-half 
natural scale. 



176 SENSITIVENESS OF THE APEX Chap. Ill 

though slight curvature from the square ; on the 23rd the tip 
had curved up above the ho izon, and on the 21th was hooked 
with the apex pointing almost to the zenith, as in Fig. 68. 

No. 6. Square attached on the 2ist ; on the 22nd slightly 
curved from the square; 23rd more curved; 25th consider- 
ably curved ; 27th all curvature lost, and the radicle was now 
directed perpendicularly downwards. 

No. 7. Square attached on the 21st ; on the 22nd a trace of 
curvature from the square, which increased next day, and on 
the 24th amounted to a right angle. 

It is, therefore, manifest that the apex of the radicle 
of the oak is highly sensitive to contact, and retains 
its sensitiveness during several days. The movement 
thus induced was, however, slower than in any of the 
previous cases, with the exception of that of ^Esculus. 
As with the bean, the terminal growing part, after 
bending, sometimes straightened itself through the 
action of geotropism, although the object still remained 
attached to the tip. 

The same remarkable experiment was next tried, 
as in the case of the bean ; namely, little squares of 
exactly the same size of the card-like sanded paper 
and of very thin paper (the thicknesses of which have 
been given under Vicia faba) were attached with 
shellac on opposite sides (as accurately as could be 
done) of the tips of 13 radicles, suspended in damp 
air, at a temperature of 65°-66° F. The result was 
striking, for 9 out of these 13 radicles became plainly, 
and 1 very slightly, curved from the thick paper 
towards the side bearing the thin paper. In two of 
these cases the apex became completely hooked after 
two days ; in four cases the deflection from the per- 
pendicular and from the side bearing the thick paper, 
amounted in from two to four days to angles of 90°, 
72°, 60°, and 49°, but in two other cases to only 18° 
and 15°. It should, however, be stated that in the 



Uhap. III. OF THE KADICLE OF ZEA. 177 

case in which the deflection was 49°, the two squares 
had accidentally come into contact on one side of the 
apex, and thus formed a lateral gable ; and the deflec- 
tion was directed in part from this gable and in part 
from the thick paper. In three cases alone the radicles 
were not affected by the difference in thickness of the 
squares of paper attached to their tips, and conse- 
quently did not bend away from the side bearing the 
stiffer paper. 

Zea mays : Sensitiveness of the apex of the Radicle to 
contact — A large number of trials were made on this 
plant, as it was the only monocotyledon on which we 
experimented. An abstract of the results will suffice. 
In the first place, 22 germinating seeds were pinned to 
cork-lids without any object being attached to their 
radicles, some being exposed to a temperature of 65 J -~ 
66° F., and others to between 74° and 79° ; and none of 
them became curved, though some were a little inclined 
to one side. A few were selected, which from having 
germinated on sand were crooked, but when suspended 
in damp air the terminal part grew straight down- 
wards. This fact having been ascertained, little squares 
of the card-like paper were affixed with shellac, on 
several occasions, to the tips of 68 radicles. Of these 
the terminal growing part of 39 became within 24 h. 
conspicuously curved away from the attached squares 
md from the perpendicular ; 13 out of the 39 forming 
Looks with their points directed towards the zenith, 
md 8 forming loops. Moreover, 7 other radicles out 
)f the 68, were slightly and two doubtfully deflected 
:om the cards. There remain 20 which were not 
iffected ; but 10 of these ought not to be counted ; 
for one was diseased, two had their tips quite sur 
rounded by shellac, and the squares on 7 had slipped 
so as to stand parallel to the apex, instead of obliquely 



178 SENSITIVENESS OF THE APEX Chap. III. 

on it. There were therefore only 10 out of the 68 
which certainly were not acted on. Some of the 
radicles which were experimented on were young and 
short, most of them of moderate length, and two or 
three exceeded three inches in length. The curva- 
ture in the above cases occurred within 24 h., but it 
was often conspicuous within a much shorter period. 
For instance, the terminal growing part of one radicle 
was bent upwards into a rectangle in 8 h. 15 m., and 
of another in 9 h. , On one occasion a hook was 
formed in 9 h. Six of the radicles in a jar containing 
nine seeds, which stood on a sand-bath, raised to 
a temperature varying from 76° to 82° F., became 
hooked, and a seventh formed a complete loop, when 
first looked at after 15 hours. 

The accompanying figures of four germinating seeds 
(Fig. 69) show, firstly, a radicle (A) the apex of which 
has become so much bent away from the attached 
square as to form a hook. Secondly (B), a hook 
converted through the continued irritation of the 
card, aided perhaps by geotropism, into an almost 
complete circle or loop. The tip in the act of forming 
a loop generally rubs against the upper part of the 
radicle, and pushes off the attached square ; the loop 
then contracts or closes, but never disappears ; and 
the apex afterwards grows vertically downwards, being- 
no longer irritated by any attached object. This 
frequently occurred, and is represented at C. The 
jar above mentioned with the six hooked radicles and 
another jar were kept for two additional days, for the 
sake of observing how the hooks would be modified. 
Most of them became converted into simple loops, 
like that figured at C ; but in one case the apex did 
not rub against the upper part of the radicle and thus 
remove the card ; and it consequently made, owing 



Ojiap. III. 



OF THE RADICLE OF Zr&. 



J 79 



to the continued irritation from the card, two complete 
loops, that is, a helix of two spires; which afterwards 
became pressed closely together. Then geotropism 
prevailed and caused the apex to grow perpendicularly 
downwards. In another case, shown at (D), the apex 



Fig. (39 







.C D. 

Zea mays: radicles excited to bend away from the little squares of card 
attached to one side of their tips. 

in making a second turn or spire, passed through the 
first loop, which was at first widely open, and in 
doing so knocked off the card ; it then grew perpen- 
dicularly downwards, and thus tied itself into a knot, 
which soon became tight ! 

Secondary Radicles of Zea. — A short time after the 
first radicle has appeared, others protrude from the 



1 80 SENSITIVENESS OF THE APEX Chap. ITL 

seed, but not laterally from the primary one. Ten of 
these secondary radicles, which were directed obliquely 
downwards, were experimented on with very small 
squares of card attached with shellac to the lower 
sides of their tips. If therefore the squares acted, the 
radicles would bend upwards in opposition to gravity. 
The jar stood (protected from light) on a sand-bath, 
which varied between 76° and 82° F. After only 
5 h. one appeared to be a little deflected from the 
square, and after 20 h. formed a loop. Four others 
were considerably curved from the squares a r ter 20 h., 
and three of them became hooked, with their tips 
pointing to the zenith, — one after 29 h. and the 
two others after 44 h. By this latter time a sixth 
radicle had become bent at a right angle from the side 
bearing the square. Thus altogether six out of the 
ten secondary radicles were acted on, four not being 
affected. There can, therefore, be no doubt that the 
tips of these secondary radicles are sensitive to slight 
contact, and that when thus excited they cause the 
upper part to bend from the touching object; but 
generally, as it appears, not in so short a time as in 
the case of the first-formed radicle. 

Sensitiveness of the tip of the Radicle to 
Moist Ant. 

Sachs made the interesting discovery, a few years 
ago, that the radicles of many seedling plants bend 
towards an adjoining damp surface.* We shall here 
endeavour to show that this peculiar form of sensitive- 
ness resides in their tips. The movement is directly 
the reverse of that excited by the irritants hitherto 
considered, which cause the growing part of the 

* ' ArLeiten des Bot. Institute in Wurztmrg,' vol. i. 1872, p. 209. 



Ohai. III. OF THE RADICLE TO MOIST AIR. 181 

radicle to bend away from the source of irritation. 
In our experiments we followed Sachs' plan, and sieves 
with seeds germinating in damp sawdust were sus- 
pended so that the bottom was generally inclined at 
40° with the horizon. If the radicles had been acted 
on solely by geotropism, they would have grown out 
of the bottom of the sieve perpendicularly down- 
wards ; but as they were attracted by the adjoining 
damp surface they bent towards it and were deflected 
50° from the perpendicular. For the sake of ascertain- 
ing whether the tip or the whole growing part of the 
radicle was sensitive to the moist air, a length of from 
1 to 2 mm. was coated in a certain number of cases 
with a mixture of olive-oil and lamp-black. This 
mixture was made in order to give consistence to the 
oil, so that a thick layer could be applied, which 
would exclude, at least to a large extent, the moist air, 
and would be easily visible. A greater number of 
experiments than those which were actually tried 
would have been necessary, had not it been clearly 
established that the tip of the radicle is the part which 
is sensitive to various other irritants. 

Phaseolus multiflorus. — Twenty-nine radicles, to which no- 
thing had been done, growing out of a sieve, were observed 
at the same time with those which had their tips greased, 
and for an equal length of time. Of the 29, 24 curved them- 
selves so as to come into close contact with the bottom of the 
sieve. The place of chief curvature was generally at a distance 
of 5 or 6 mm. from the apex. Eight radicles had their tips 
greased for a length of 2 mm., and two others for a length of 
U mm. ; they were kept at a temperature of 15°-16° C. After 
intervals of from 19 h. to 24 h. all were still vertically or 
almost vertically dependent, for some of them had moved 
towards the adjoining damp surface by about 10°. They had 
therefore not been acted on, or only slightly acted on, by the 
damper air on one side, although the whole upper part was 
freely exposed. After 48 h. three of these radicles became 



182 SENSITIVENESS OF THE APEX Cijap. III. 

considerably curved towards the sieve ; and the absence of curva- 
ture in some of the others might perhaps be accounted for by 
their not having grown very well. But it should be observed 
that during the first 19 h. to 24 h. all grew well ; two of them 
having increased 2 and 3 mm. in length in 11 h. ; five others 
increased 5 to 8 mm. in 19 h. ; and two, which had been at first 
4 and 6 mm. in length, iL creased in 24 h. to 15 and 20 mm. 

The tips of 10 radicles, which likewise grew well, were coated 
with the grease for a length of only 1 mm., and now the result 
was some^v hat different ; for of these 4 curved themselves to 
the sieve in from 21 h. to 24 h., whilst 6 did not do so. 
Five of the latter were observed for an additional day, and now 
all excepting one became curved to the sieve. 

The tips of 5 radicles were cauterised with nitrate of silver, 
and about 1 mm. in length was thus destroyed. They were 
observed for periods varying between 11 h. and 24 h., and were 
found to have grown well. One of them had curved until it 
came into contact with the sieve ; another was curving towards 
it ; whilst the remaining three were still vertically dependent. 
Of 7 not cauterised radicles observed at the same time, all had 
come into contact with the sieve. 

The tips of 11 radicles were protected by moistened gold- 
beaters' skin, which adheres closely, for a length varying from 
lh to 2 2 mm. After 22 h. to 24 h, 6 of these radicles were 
clearly bent towards or had come into contact with the sieve ; 
2 were slightly curved in this direction, and 3 not at all. All 
had grown well. Of 14 control specimens observed at the same 
time, all excepting one had closely approached the sieve. It 
appears from these cases that a cap of goldbeaters' skin checks, 
though only to a slight degree, the bending of the radicles to 
an adjoining damp surface. Whether an extremely thin sheet 
of this substance when moistened allows moisture from the air 
to pass through it, we do not know. One case indicated that 
the caps were sometimes more efficient than appears from the 
above results; for a radicle, which after 23 h. had only 
slightly approached the sieve, had its cap (U mm. in length) 
removed, and during the next 15 1 h. it curved itself abruptly 
towards the source of moisture, the chief seat of curvature 
being at a distance of 2 to 3 mm. from the apex. 

Vicia faba. — The tips of 13 radicles were coated with the 
grease for a length of 2 mm. ; and it should be remembered 
that with these radicles the seat of chief curvature is about 



Chap. III. OF THE RADICLE TO MOIST AIR. 183 

4 or 5 mm. from the apex. Four of them were examined after 
22 h., three after 26 h., and six after 36 h., and none had 
been attracted towards the damp lower surface of the sieve. 
In another trial 7 radicles were similarly treated, and 5 of them 
still pointed perpendicularly downwards after 11 h., whilst 

2 were a little curved towards the sieve; by an accident they 
were not subsequently observed. .In both these trials the 
radicles grew well ; 7 of them, which were at first from 4 to 
11 mm. in length, were after 11 h. between 7 and 16 mm. ; 

3 which were at first from 6 to 8 mm. after 26 h. were 11*5 
to 18 mm. in length ; and lastly, 4 radicles which were at first 

5 to 8 mm. after 46 h. were 18 to 23 mm. in length. The 
control or ungreased radicles were not invariably attracted 
towards the bottom of the sieve. But on one occasion 12 out of 
13, which were observed for periods between 22 h. and 36 h., 
were thus attracted. On two other occasions taken together, 
38 out of 40 were similarly attracted. On another occasion 
only 7 out of 14 behaved in this manner, but after two more 
days the proportion of the curved increased to 17 out of 23. 
On a last occasion only 11 out of 20 were thus attracted. If 
we add up these numbers, we find that 78 out of 96 of the 
control specimens curved themselves towards the bottom of the 
sieve. Of the specimens with greased tips, 2 alone out of the 
20 (but 7 of these were not observed for a sufficiently long 
time) thus curved themselves. We can, therefore, hardly doubt 
that the tip for a length of 2 mm. is the part which is sensitive 
to a moist atmosphere, and causes the upper part to bend 
towards its source. 

The tips of 15 radicles were cauterised with nitrate of silver, 
and they grew as well as those above described with greased 
tips. After an interval of 24 h., 9 of them were not at all 
curved towards the bottom of the sieve ; 2 were curved towards 
it at angles of 20° and 12° from their former vertical position, 
and 4 had come into close contact with it. Thus the destruc- 
tion of the tip for a length of about 1 mm. prevented the curva- 
ture of the greater number of these radicles to the adjoining 
damp surface. Of 24 control specimens, 23 were bent to the 
sieve, and on a second occasion 15 out of 16 were similarly 
curved in a greater or less degree. These control trials are 
included in those given in the foregoing paragraph. 

Avena sati.va. — The tips of 13 radicles, which projected 
between 2 and 4 mm. from the bottom of the sieve, many of 
13 



181 SENSITIVENESS OF THE APEX Chap. TIL 

them not quite perpendicularly downwards, were coated with 
the black grease for a length of from 1 to li mm. The sieves 
were inclined at 30° with the horizon. The greater number of 
these radicles were examined after 22 h., and a few after 25 h., 
and within these intervals they had grown so quickly as to have 
nearly doubled their lengths. With the ungreased radicles the 
chief seat of curvature is at a distance of not less than between 
35 and 55 mm., and not more than between 7 and 10 mm. from 
the ajDex. Out of the 13 radicles with greased tips, 4 had not 
moved at all towards the sieve ; 6 were deflected towards it and 
from the perpendicular by angles varying between 10° and 35° ; 
and 3 had come into close contact with it. It appears, therefore, 
at first sight that greasing the tips of these radicles had checked 
but little their bending to the adjoining damp surface. But the 
inspection of the sieves on two occasions produced a widely 
different impression on the mind; for it was impossible to 
behold the radicles with the black greased tips projecting from 
the bottom, and all those with ungreased tips, at least 40 to 50 
in number, clinging closely to it, and feel any doubt that the 
greasing had produced a great effect. On close examination 
only a single ungreased radicle could be found which had not 
become curved towards the sieve. It is probable that if the 
tips had been protected by grease for a length of 2 mm. instead 
of from 1 to Is mm., they would not have been affected by the 
moist air and none would have become curved. 

Triticum v/dgare. — Analogous trials were made on 8 radicles 
of the common wheat ; and greasing their tips produced much 
less effect than in the case of the oats. After 22 h., 5 of them 
had come into contact with the bottom of the sieve; 2 had 
moved towards it 10° and 15°, and one alone remained perpen- 
dicular. Not one of the very numerous ungreased radicles 
failed to come into close contact with the sieve. These trials 
were made on Nov. 28th, when the temperature was only 4° - 8 C. 
at 10 a.m. We should hardly have thought this case worth 
notice, had it not been for the following circumstance. In the 
beginning of October, when the temperature was considerably 
higher, viz., 12° to 13° C, we found that only a few of the 
ungreased radicles became bent towards the sieve ; and this 
indicates that sensitiveness to moisture in the air is increased 
by a low temperature, as we have seen with the radicles of 
Vicia faba relatively to objects attached to their tips. But iD 
the present instance it is possible that a difference in the dryness 



Chap. III. OF THE RADICLE TO MOIST AIR. ISt 

of the air may have caused the difference in the results at the 
two periods. 

Finally, the facts just given with respect to Phaseohts 
multijiorus, Vicia fala, and Avena sativa show, as ii 
seems to us, that a layer of grease spread for a length 
of li to 2 mm. over the tip of the radicle, or the 
destruction of the tip by caustic, greatly lessens or 
quite annuls in the upper and exposed part the power 
of bending towards a neighbouring source of moisture. 
We should bear in mind that the part which bends 
most, lies at some little distance above the greased or 
oauterised tip ; and that the rapid growth of this part, 
proves that it has not been injured by the tips having 
been thus treated. In those cases in which the radicles 
with greased tips became curved, it is possible that the 
layer of grease was not sufficiently thick wholly to ex- 
clude moisture, or that a sufficient length was not thus 
protected, or, in the case of the caustic, not destroyed. 
When radicles with greased tips are left to grow for 
several days in damp air, the grease is drawn out into 
the finest reticulated threads and dots, with narrow 
portions of the surface left clean. Such portions 
would, it is probable, be able to absorb moisture, and 
thus we can account for several of the radicles with 
greased tips having become curved towards the sieve 
after an interval of one or two days. On the whole, 
we may infer that sensitiveness to a difference in the 
amount of moisture in the air on the two sides of a 
radicle resides in the tip, which transmits some influ- 
ence to the upper part, causing it to bend towards the 
source of moisture. Consequently, the movement is 
the reverse of that caused by objects attached to one 
side of the tip, or by a thin slice being cut off, or by 
being slightly cauterised. In a future chapter it 
will be shown that sensitiveness to the attraction of 



186 THE EFFECT OF KILLING OR Chap. 111. 

gravity likewise resides in the tip; so that it is the 
tip which excites the adjoining parts of a horizontally 
extended radicle to bend towards the centre of the 
earth. 

Secondary Kadicles becoming vertically Geo- 
tropic by the destruction or injury of the 
Terminal Part of the Primary Eadicle. 

Sachs has shown that the lateral or secondary 
radicles of the bean, and probably of other plants, are 
acted on by geotropism in so peculiar a manner, that 
they grow out horizontally or a little inclined down- 
wards ; and he has further shown* the interesting fact, 
that if the end of the primary radicle be cut off, one 
of the nearest secondary radicles changes its nature 
and grows perpendicularly downwards, thus replacing 
the primary radicle. We repeated this experiment, 
and planted beans with amputated radicles in friable 
peat, and saw the result described by Sachs ; but 
generally two or three of the secondary radicles grew 
perpendicularly downwards. We also modified the 
experiment, by pinching young radicles a little way 
above their tips, between the arms of a U-shaped 
piece of thick leaden wire. The part pinched was 
thus flattened, and was afterwards prevented from 
growing thicker. Five radicles had their ends cut 
off, and served as controls or standards. Eight were 
pinched ; of these 2 were pinched too severely and 
their ends died and dropped off ; 2 were not pinched 
enough and were not sensibly affected ; the remaining 
4 were pinched sufficiently to check the growth ot 
the terminal part, but did not appear otherwise injured. 
When the U-shaped wires were removed, after an 



Arbeiten Bot. Institut., Wiirzburg,' Heft iv. 1874, p. 622. 



Obxv. Ill, INJURING THE PRIMARY RADICLE. 187 

interval of 15 days, the part beneath the wire was 
found to be very thin and easily broken, whilst the 
part above was thickened. Now in these fonr cases, 
one or more of the secondary radicles, arising from 
the thickened part just above the wire, had grown 
perpendicularly downwards. In the best case the 
primary radicle (the part below the wire being 1J inch 
in length) was somewhat distorted, and was not half 
as long as three adjoining secondary radicles, which 
had grown vertically, or almost vertically, downwards. 
Some of these secondary radicles adhered together or 
had become confluent. We learn from these four cases 
that it is not necessary, in order that a secondary 
radicle should assume the nature of a primary one, 
that the latter should be actually amputated ; it is 
sufficient that the flow of sap into it should be 
checked, and consequently should be directed into the 
adjoining secondary radicles ; for this seems to be 
the most obvious result of the primary radicle being 
pinched between the arms of a U-shaped wire. 

This change in the nature of secondary radicles is 
dearly analogous, as Sachs has remarked, to that 
which occurs with the shoots of trees, when the leading 
one is destroyed and is afterwards replaced by one or 
more of the lateral shoots ; for these now grow upright 
instead of sub-horizontally. But in this latter case 
the lateral shoots are rendered apogeotropic, whereas 
with radicles the lateral ones are rendered geotropic. 
We are naturally led to suspect that the same cause 
acts with shoots as with roots, namely, an increased flow 
of sap into the lateral ones. We made some trials with 
Abies communis and peetinata, by pinching with wire 
the leading and all the lateral shoots excepting one. 
Rut we believe that they were too old when experi- 
mented on ; and some were pinched too severely, and 



188 THE EFFECT OF KILLING OR Chap, in 

some not enough. Only one case succeeded, namely 
with the spruce-fir. The leading shoot was not killed, 
but its growth was checked ; at its base there were 
three lateral shoots in a whorl, two of which were 
pinched, one being thus killed ; the third was left 
untouched. These lateral shoots, when operated on 
(July 14th) stood at an angle of 8° above the horizon ; 
by Sept. 8th the unpinched one had risen 35° ; by 
Oct. 4th it had risen 46°, and by Jan. 26th 48°, and 
it had now become a little curved inwards. Part 
of this rise of 48° may be attributed to ordinary 
growth, for the pinched shoot rose 12° within the same 
period. It thus follows that the unpinched shoot 
stood, on Jan. 26th, 56° above the horizon, or 34° 
from the vertical; and it was thus obviously almost 
ready to replace the slowly growing, pinched, lead- 
ing shoot. Nevertheless, we feel some doubt about 
this experiment, for we have since observed with 
spruce-firs growing rather unhealthily, that the lateral 
shoots near the summit sometimes become highly 
inclined, whilst the leading shoot remains apparently 
sound. 

A widely different agency not rarely causes shoots 
which naturally would have grown out horizontally to 
grow up vertically. The lateral branches of the Silver 
Fir (A. pectinata) are often affected by a fungus, 
JEcidium elatinum, which causes the branch to enlarge 
into an oval knob formed of hard wood, in one of 
which we counted 24 rings of growth. According to 
De Bary,* when the mycelium penetrates a bud be- 
ginning to elongate, the shoot developed from it 
grows vertically upwards. Such upright shoots after- 



* See his valuable article in are culled in Gorman " He xen 
'Bot. Zeitung,' 18(37, p. 257, on besen," or " witch-brooms." 
these monstrous growths, which 



Chap. III. INJURING THE PRIMARY RADICLE. 189 

-ards produce lateral and horizontal branches; and 
they then present a curious appearance, as if a youug 
fir- tree had grown out of a ball of clay surrounding 
the branch. These upright shoots have manifestly 
changed their nature and become apogeotropic ; for if 
they had not been affected by the iEcidium, they 
would have grown out horizontally like all the other 
twigs on the same branches. This change can hardly 
be due to an increased flow of sap into the part ; but 
the presence of the mycelium will have greatly dis- 
turbed its natural constitution. 

According to Mr. Meehan,* the stems of three 
species of Euphorbia and of Portulaca oleraeea are 
" normally prostrate or procumbent ;" but when they 
are attacked by an iEciclium, they " assume an erect 
habit." Dr. Stahl informs us that he knows of several 
analogous cases ; and these seem - to be closely related 
to that of the Abies. The rhizomes of Sparganium 
ramosum grow out horizontally in the soil to a con- 
siderable length, or are diageotropic ; but F. Elfving 
found that when they were cultivated in water 
their tips turned upwards, and they became apogeo- 
tropic. The same result followed when the stem of the 
plant was bent until it cracked or was merely much 
bowed.f 

No explanation has hitherto been attempted of such 
cases as the foregoing, — namely, of secondary radiclss 
growing vertically downwards, and of lateral shoots 
growing vertically upwards, after the amputation of 



* ' Proc. Acad. Nat. Sc. Phil a- viously observed (' Flora,' 1878, 

dfclphia,' June 16th, 1874, and p. 324) that the underground 

July 23rd, 1875. shoots of Triticum repens bend 

t See F. Elfving's interesting vertically up when the parts above 

paper in 'Arbeiten Bot. Institut., ground are removed, aud when 

in Wurzburg,' vol. ii. 1880, p. 489. tho rhizomes are kept partly im 

Carl Kraus (Triesdorf) had pre- mcrsul in water. 



I 90 EFFECT OF KILLING PRIMARY KADICLE. Chap. IIL 

the primary radicle or of the leading shoot. The 
following considerations give us, as we believe, the 
clue. Firstly, any cause which disturbs the con- 
stitution * is apt to induce reversion ; such as the 
crossing of two distinct races, or a change of con- 
ditions, as when domestic animals become feral. 
But the case which most concerns us, is the frequent 
appearance of peloric flowers on the summit of a stem, 
or in the centre of the inflorescence, — parts which, it is 
believed, receive the most sap ; for when an irregular 
flower becomes perfectly regular or peloric, this may 
be attributed, at least partly, to reversion to a primi- 
tive and normal type. Even the position of a seed at 
the end of the capsule sometimes gives to the seedling 
developed from it a tendency to revert. Secondly, 
reversions often occur by means of buds, independently 
of reproduction by seed ; so that a bud may revert to 
the character of a former state many bud-generations 
ago. In the case of animals, reversions may occur in 
the individual with advancing age. Thirdly and 
lastly, radicles when they first protrude from the seed 
are always geotropic, and plumules or shoots almost 
always apogeotropic. If then any cause, such as an 
increased flow of sap or the presence of mycelium, 
disturbs the constitution of a lateral shoot or of a 
secondary radicle, it is apt to revert to its primordial 
state ; and it becomes either, apogeotropic or geotropic, 
as the case may be, and consequently grows either 
vertically upwards or downwards. It is indeed pos- 



* The facts on which the fol- xiv. On pyloric flowers, chap, 

lowing conclusions are founded xid. p. 32 ; and see p. 337 on theii 

are given in ' The Variation of position on the plant. With 

Animals and Plants under Domes- respect to seeds, p. 340. On re- 

tiration,' 2nd edit 1875. On the version by means of buds, p. 438 

causes leading to reversi >n see chap, xi vol. i. 
chnp. xii. vol. ii. and p. 59 chap. 



Chap. III. SUMMARY OF CHAPTER. 191 

sible, or even probable, that this tendency to reversion 
may have been increased, as it is manifestly of servicu 
to the plant. 

Summary of Chapter. 

A part or organ may be called sensitive, when its 
irritation excites movement in an adjoining part. Now 
it has been show 7 n in this chapter, that the tip of the 
radicle of the bean is in this sense sensitive to the 
contact of any small object attached to one side by 
shellac or gum-water ; also to a slight touch with dry 
caustic, and to a thin slice cut off one side. The 
radicles of the pea were tried with attached objects 
and caustic, both of which acted. With Pliaseolus 
multiflorus the tip was hardly sensitive to small squares 
of attached card, but was sensitive to caustic and to 
slicing. The radicles of Tropaeolum were highly sen- 
sitive to contact ; and so, as far as we could judge, 
were those of Gossypium herbaceum, and they were 
certainly sensitive to caustic. The tips of the radicles 
of Gucurbita ovifera were likewise highly sensitive to 
caustic, though only moderately so to contact. Ea- 
phanus sativus offered a somewhat doubtful case. 
With iEsculus the tips were quite indifferent to 
bodies attached to them, though sensitive to caustic. 
Those of Quercus nibur and Zea mays were highly sen- 
sitive to contact, as were the radicles of the latter 
to caustic. In several of these cases the difference in 
sensitiveness of the tip to contact and to caustic was, 
as we believe, merely apparent ; for with Gossypium, 
Kaphanus, and Cucurbita, the tip was so fine and 
flexible that it was very difficult to attach any object 
to one of its sides. With the radicles of iEsculus, 
the tips were not at all sensitive to small bodies 
attached to them ; but it does not follow from this 



L92 SUMMARY OF CHAPTER. Chap. Ill 

fact that they would not have been sensitive to some- 
what greater continued pressure, if this could have 
been applied. 

The peculiar form of sensitiveness which we are 
here considering, is confined to the tip of the radicle 
for a length of from 1 mm. to 1 • 5 mm. When this 
part is irritated by contact with any object, by caustic, 
or by a thin slice being cut off, the upper adjoining 
part of the radicle, for a length of from 6 or 7 to 
even 12 mm., is excited to bend away from the side 
which has been irritated. Some influence must there- 
fore be transmitted from the tip along the radicle for 
this length. The curvature thus caused is generally 
symmetrical. The part which bends most apparently 
coincides with that of the most rapid growth. The 
tip and the basal part grow very slowly and they 
bend very little. 

Considering the widely separated position in the 
vegetable series of the several above-named genera, 
we may conclude that the tips of the radicles of all, or 
almost all, plants are similarly sensitive, and transmit 
an influence causing the upper part to bend. With 
respect to the tips of the secondary radicles, those of 
Vicia fdba, Pisum sativum, and Zea mays were alone 
observed, and they were found similarly sensitive. 

In order that these movements should be properly 
displayed, it appears necessary that the radicles 
should grow at their normal rate. If subjected to a 
high temperature and made to grow rapidly, the 
tips seem either to lose their sensitiveness, or the 
upper part to lose the power of bending. So it 
appears to be if they grow very slowly from not being 
vigorous, or from being kept at too low a temperature , 
also when they are forced to germinate in the middle 
of the winter. 



Chai III. SUMMARY OF CHAPTER. 193 

The curvature of the radicle sometimes occurs 
within from 6 to 8 hours after the tip has been irritated, 
and almost always within 24 h., excepting in the 
case of the massive radicles of iEsculus. The curva- 
ture often amounts to a rectangle, — that is, the ter- 
minal part bends upwards until the tip, which is but 
little curved, projects almost horizontally. Occa- 
sionally the tip, from the continued irritation of the 
attached object, continues to bend up until it forms a 
hook with the point directed towards the zenith, or 
a loop, or even a spire. After a time the radicle 
apparently becomes accustomed to the irritation, as 
occurs in the case of tendrils, for it again grows down- 
wards, although the bit of card or other object may 
remain attached to the tip. 

It is evident that a small object attached to the free 
point of a vertically suspended radicle can offer no 
mechanical resistance to its growth as a whole, for the 
object is carried downwards as the radicle elongates, 
or upwards as the radicle curves upwards. Nor can 
the growth of the tip itself be mechanically checked 
by an object attached to it by gum- water, which 
remains all the time perfectly soft. The weight of 
the object, though quite insignificant, is opposed 
to the upward curvature. We may therefore conclude 
that it is the irritation due to contact which excites 
the movement. The contact, however, must be pro- 
longed, for the tips of 15 radicles were rubbed for a 
,! hort time, and this did not cause them to bend. Here 
then we have a case of specialised sensibility, like 
that of the glands of Drosera ; for these are ex- 
quisitely sensitive to the slightest pressure if prolonged, 
but not to two or three rough touches. 

When the tip of a radicle is lightly touched on one 
side with dry nitrate of silver, the injury caused is 



L9i SUMMARY OF CHAPTER. Chap. III. 

very slight, and the adjoining upper part bends away 
from the cauterised point, with more certainty in most 
cases than from an object attached on one side. Here 
it obviously is not the mere touch, but the effect 
produced by the caustic, which induces the tip to 
transmit some influence to the adjoining part, causing 
it to bend away. If one side of the tip is badly 
injured or killed by the caustic, it ceases to grow, 
whilst the opposite side continues growing ; and the 
result is that the tip itself bends towards the injured 
side and often becomes completely hooked ; and it is 
remarkable that in this case the adjoining upper part 
does not bend. The stimulus is too powerful or the 
shock too great for the proper influence to be trans- 
mitted from the tip. We have strictly analogous cases 
with Drosera, Dionaea and Pinguicula, with which 
plants a too powerful stimulus does not excite the 
tentacles to become incurved, or the lobes to close, or 
the margin to be folded inwards. 

With respect to the degree of sensitiveness of the 
apex to contact under favourable conditions, we have 
seen that with Vicia fdba a little square of writing- 
paper affixed with shellac sufficed to cause move- 
ment; as did on one occasion a square of merely 
damped goldbeaters' skin, but it acted very slowly. 
Short bits of moderately thick bristle (of which mea- 
surements have been given) affixed with gum-water 
acted in only three out of eleven trials, and beads oi 
dried shellac under 20 o^ n °f a g ram m weight acted 
only twice in nine cases ; so that here we have 
nearly reached the minimum of necessary irrita- 
tion. The apex, therefore, is much less sensitive to 
pressure than the glands of Drosera, for these are 
affected by far thinner objects than bits of bristle 
and by a very much less weight than 2 ^ 5 th of a grain. 



Chap. III. SUMMARY OF CHAPTER. 195 

But the most interesting evidence of the delicate 
sensitiveness of the tip of the radicle, was afforded by 
its power of discriminating between equal-sized squares 
of card-like and very thin paper, when these were 
attached on opposite sides, as was observed with the 
radicles of the bean and oak. 

When radicles of the bean are extended horizon- 
tally with squares of card attached to the lower sides of 
their tips, the irritation thus caused was always con- 
quered by geotropism, which then acts under the most 
favourable conditions at right angles to the radicle. 
But when objects were attached to the radicles of and 
of the above-named genera, suspended vertically, the 
irritation conquered geotropism, which latter power 
at first acted obliquely on the radicle ; so that the 
immediate irritation from the attached object, aided 
by its after-effects, prevailed and caused the radicle 
to bend upwards, until sometimes the point was 
directed to the zenith. We must, however, assume 
that the after-effects of the irritation of the tip by an 
attached object come into play, only after movement 
has been excited. The tips of the radicles of the pea 
seem to be more sensitive to contact than those of the 
bean, for when they were extended horizontally with 
squares of card adhering to their lower sides, a most 
curious struggle occasionally arose, sometimes one 
and sometimes the other force prevailing, but ulti- 
mately geotropism was always victorious ; neverthe- 
less, in two instances the terminal part became so 
much curved upwards that loops were subsequently 
formed. With the pea, therefore, the irritation from 
an attached object, and from geotropism when acting 
at right angles to the radicle, are nearly balanced 
forces. Closely similar results were observed with the 
horizontally extended radicles of Cuourbita ovifera^ 



196 SUMMARY OF CHAPTER. Chap. Ill, 

when their tips were slightly cauterised on the lower 
side. 

Finally, the several co-ordinated movements by 
which radicles are enabled to perform their proper 
functions are admirably perfect. In whatever direc- 
tion the primary radicle first protrudes from the seed, 
geotropism guides it perpendicularly downwards ; and 
the capacity to be acted on by the attraction of 
gravity resides in the tip. But Sachs has proved* 
that the secondary radicles, or those emitted by the 
primary one, are acted on by geotropism in such a 
manner that they tend to bend only obliquely down- 
wards. If they had been acted on like the primary 
radicle, all the radicles would have penetrated the 
ground in a close bundle. We have seen that if 
the end of the primary radicle is cut off or in- 
jured, the adjoining secondary radicles become geo- 
tropic and grow vertically downwards. This power 
must often be of great service to the plant, when the 
primary radicle has been destroyed by the larvae of 
insects, burrowing animals, or any other accident. The 
tertiary radicles, or those emitted by the secondary 
ones, are not influenced, at least in the case of the 
bean, by geotropism ; so they grow out freely in all 
directions. From this manner of growth of the various 
kinds of radicles, they are distributed, together with 
their absorbent hairs, throughout the surrounding soil, 
as Sachs has remarked, in the most advantageous 
manner ; for the whole soil is thus closely searched. 

Geotropism, as was shown in the last chapter, 
excites the primary radicle to bend downwards with 
very little force, quite insufficient to penetrate the 
ground. Such penetration is effected by the pointed 



Ar'ieiten Bot. lnstitut., Wurzburg,' Heft iv. 1874, pp. 605-631 



Chap. in. SUMMARY OF CHAPTER. 197 

apex (protected by the root-cap) being pressed down 
by the longitudinal expansion or growth of the ter- 
minal rigid portion, aided by its transverse expan- 
sion, both of which forces act powerfully. It is, 
however, indispensable that the seeds should be at 
first held down in some manner. When they lie 
on the bare surface they are held down by the attach- 
ment of the root-hairs to any adjoining objects ; and 
this apparently is effected by the conversion of 
their outer surfaces into a cement. But many seeds 
get covered up by various accidents, or they fall into 
crevices or holes. With some seeds their own weight 
suffices. 

The circumnutating movement of the terminal grow- 
ing part both of the primary and secondary radicles 
is so feeble that it can aid them very little in pene- 
trating the ground, excepting when the superficial 
layer is very soft and damp. But it must aid them 
materially when they happen to break obliquely into 
cracks, or into burrows made by earth-worms or larvae. 
This movement, moreover, combined with the sen- 
sitiveness of the tip to contact, can hardly fail to be 
of the highest importance ; for as the tip is always 
endeavouring to bend to all sides it will press on all 
sides, and will thus be able to discriminate between 
the harder and softer adjoining surfaces, in the same 
manner as it discriminated between the attached 
squares of card-like and thin paper. Consequently it 
will tend to bend from the harder soil, and will thus 
follow the lines of least resistance. So it will be if it 
meets with a stone or the root of another plant in the 
soil, as must incessantly occur. If the tip were not 
sensitive, and if it did not excite the upper part of the 
root to bend away, whenever it encountered at right 
angles some obstacle in the ground, it would be liable 



198 SUMMAKY OF CHAPTER. Chap. IJI 

to be doubled up into a contorted mass. But we have 
seen with, radicles growing down inclined plates oi 
glass, that as soon as the tip merely touched a slip oi 
wood cemented across the plate, the whole terminal 
growing part curved away, so that the tip soon stood 
at right angles to its former direction ; and thus it 
would be with an obstacle encountered in the ground, 
as far as the pressure of the surrounding soil would 
permit. We can also understand why thick and strong 
radicles, like those of ^Esculus, should be endowed 
with less sensitiveness than more delicate ones ; for 
the former would be able by the force of their growth 
to overcome any slight obstacle. 

After a radicle, which has been deflected by some 
stone or root from its natural downward course, 
reaches the edge of the obstacle, geotropism will direct 
it to grow again straight downward ; but we know that 
geotropism acts with very little force, and here another 
excellent adaptation, as Sachs has remarked,* comes 
into play. For the upper part of the radicle, a little 
above the apex, is, as we have seen, likewise sensitive ; 
and this sensitiveness causes the radicle to bend like a 
tendril towards the touching object, so that as it rubs 
over the edge of an obstacle, it will bend downwards ; 
and the curvature thus induced is abrupt, in which 
respect it differs from that caused by the irritation of 
one side of the tip. This downward bending coincides 
with that due to geotropism, and both will cause the 
root to resume its original course. 

As radicles perceive an excess of moisture in the air 
on one side and bend towards this side, we may infer 
that they will act in the same manner with respect to 
moisture in the earth. The sensitiveness to moisture 



* ' Aibeiten Bot. Inst. Wiirzburg.' Heft iii. p. 456. 



Chap. IIL SUMMARY UF CHAPTER. 199 

resides in the tip, which determines the bending of 
the upper part. This capacity perhaps partly accounts 
for the extent to which drain-pipes often become 
choked with roots. 

Considering the several facts given in this chapter, 
we see that the course followed by a root through 
the soil is governed by extraordinarily complex and 
diversified agencies, — by geotropism acting in a 
different manner on the primary, secondary, and ter- 
tiary radicles, — by sensitiveness to contact, different in 
kind in the apex and in the part immediately above 
the apex, and apparently by sensitiveness to the 
varying dampness of different parts of the soil. 
These several stimuli to movement are all more 
powerful than geotropism, when this acts obliquely 
on a radicle, which has been deflected from its perpen- 
dicular downward course. The roots, moreover, of 
most plants are excited by light to bend either to or 
from it ; but as roots are not naturally exposed to the 
light it is doubtful whether this sensitiveness, which is 
perhaps only the indirect result of the radicles being 
highly sensitive to other stimuli, is of any service to 
the plant. The direction which the apex takes at each 
successive period of the growth of a root, ultimately 
determines its whole course ; it is therefore highly 
important that the apex should pursue from the first 
the most advantageous direction ; and we can thus 
understand why sensitiveness to geotropism, to contact 
and to moisture, all reside in the tip, and why the tip 
determines the upper growing part to bend either 
from or to the exciting cause. A radicle may be 
compared w r ith a burrowing animal such as a mole, 
which wishes to penetrate perpendicularly down into 
the ground. By continually moving his head from 
side to side, or circumnutating, he will feel any stone 
14 



200 SUMMARY OF CHArTEK. Chat.' IIL 

or other obstacle, as well as any difference in the 
hardness of the soil, and he will turn from that side ; 
if the earth is damper on one than on the other side 
he will turn thitherward as a Letter hunting-ground. 
Nevertheless, after each interruption, guided by the 
sense of gravity, he will be able to recover his down- 
ward course and to burrow to a greater depth. 



Chap. IV. GIRCUMNUTATIOH 201 



CHAPTER IV. 

The Circtjmnttatixg Movements of the severa_ parts of 
Mature Plants. 

Circumnutation of stems : concluding remarks on— Circumnutation of 
stolons: aid thus afforded, in winding amongst the stems of sur- 
rounding plants — Circumnutation of flower-stems — Circumnutation 
of Dicotyledonous leaves — Singular oscillatory movement of leaves 
of Diontea— Leaves of Cannabis sink at night — Leaves of Gymno- 
sperms — Of Monocotyledons — Cryptogams — Concluding remarks 
on the circumnut ition of leaves : generally rise in the evening and 
sink in the morning. 

We have seen in the first chapter that the stems of all 
seedlings, whether hypocotyls or epicotyls, as well as 
the cotyledons and the radicles, are continually cir- 
cumnutating — that is, they grow first on one side and 
then on another, such growth being probably preceded 
by increased turgescence of the cells. As it was 
unlikely that plants should change their manner of 
growth with advancing age, it seemed probable that 
the various organs of all plants at all ages, as long as 
they continued to grow, would be found to circum- 
nutate, though perhaps to an extremely small extent. 
As it was important for us to discover whether this 
was the case, we determined to observe carefully a 
certain number of plants which were growing vigor- 
ously, and which were not known to move in any 
manner. We commenced with stems. Observations 
of this kind are tedious, and it appeared to us that it 
would be sufficient to observe the stems in about a 
score of genera, belonging to widely distinct families 
and inhabitants of various countries. Several plants 



202 CIKCUMNUTATION OF STEMS. Chap. IV 

were selected which, from being woody, or for other 
reasons, seemed the least likely to circumnutate. The 
observations and the diagrams were made in the 
manner described in the Introduction. Plants in pots 
were subjected to a proper temperature, and whilst 
being observed, were kept either in darkness or were 
feebly illuminated from above. They are arranged 
in the order adopted by Hooker in Le Maout and 
Decaisne's ' System of Botany.' The number of the 
family to. which each genus belongs is appended, as 
this serves to show the place of each in the series. 

(1.) Iberia umbellata (Cruciferse, Fam. 14).— The movement of 
the stem of a young plant, 4 inches in height, consisting of 
four internodes (the hypocotyl included) besides a large bud 

Fis: 70. 




Iberis umbellata : circumnutation of stem of young plant, traced from 
8.30 a.m. Sept. 13th to same hour on following morning. Distance of 
summit of stem beneath the horizontal glass 7*6 inches. Diagram 
reduced to half of original size. Movement as here shown magnifie I 
between 4 and 5 times. 

on the summit, was traced, as here shown, during 24 h. 
(Fig. 70). As far as we could judge the uppermost inch alone 
of the stem circumnutated, and this in a simple manner. The 
movement was slow, and the rate very unequal at different 
times. In part of its course an irregular ellipse, or rather 
triangle, was completed in 6 h. 30 in. 

(2.) Brassica oleracea (Ouciferse). — A very young plant, bearing 
three leaves, of which the longest was only three-quarters of arj 
inch in length, was placed under a microscope, furnished with 
an eye-piece micrometer and the tip of the largest leaf was 



Chap. IV. CIKCUMNUTATION OF STEMS. 203 

found to be in constant movement. It crossed five di visions of 
the micrometer, that is, y^h of an inch, in 6 m. 20 s. There 
could hardly be a doubt that it was the stem which chiefly 
moved, for the tip did not get quickly out of focus ; and this 
would have occurred had the movement been confined to the 
leaf, which moves up or down in nearly the same vertical plane. 

(3.) Linum usitatissimum (Linese, Earn. 39). — The stems of this 
plant, shortly before the flowering period, are stated by Fritz 
Miiller ('Jenaische Zeitschrift,' B. v. p. 137) to revolve, or 
circumnutate. 

(4.) Felargonium zonah (Geraniaceae, Fam. 47).— A young 
plant, 7h inches in height, was observed in the usual manner ; 
but, in order to see the bead at the end of the glass filament 



Fig. 71 



lO'20p.m.Q A 




/\fl*wJ# 



<10'3Q'pjnJO th 

Pelargonium zonale: circumnutation of stem of young plant, feebly illu- 
minated from above. Movement of bead magnified about 11 times ; 
traced on a horizontal glass from noon on March 9th to 8 A.M. on 
the 11th. 

and at the same time the mark beneath, it was necessary to cut 
off three leaves on one side. We do not know whether it was 
owing to this cause, or to the plant having previously become 
bent to one side through heliotropism, but from the morning of 
the 7th of March to 10.30 p.m. on the 8th, the stem moved 
a considerable distance in a zigzag line in the same general 
direction. During the night of the 8th it moved to some 
distance at right angles to its former course, and next morning 
(9th) stood for a time almost still. At noon on the 9th a new 
tracing was begun (see Fig. 71), which was continued till 8 a.m. 
ou the 11th. Between noon on the 9th and 5 p.m. on the 10th 
(i.e. in the course of 29 h.), the stem described a circle. This 
plant therefore circumnutates, but at a very slow rate, and to a 
small extent. 

(5.) Tropceolum majus (?) (dwarfed var. called Tom Thumb) ; 
(Geraniacesc, Fam. 47). — The species of this genus climb by the 



204 



CIRCUMNUTATION OF STEMS. 



CUAP- 17 



aid of their sensitive petioles, but some of them also twint 
round supports; but even these latter species do not begin to 
circumnutate in a conspicuous manner whilst young. The 

Fig. 72. 




"> 

Trojxzolum majus(?): circumnutation of stem of young plant, traced on a 
horizontal glass from 9 A.M. Dec. 26th to 10 a.m. on 27th. Movement 
of bead magnified about 5 times, and here reduced to half of original 
scale. 

variety here treate 1 of has a rather thick stem, and is so dwarf 
that apparently it does not climb in any manner. We there- 
fore wished to ascertain whether the stem of a young plant, 

consisting of two in- 
ternodes, together 3*2 
inches in height, cir- 
cumnutated. It was 
observed during 25 h., 
and we see in Fig. 72 
that the stem moved in 
a zigzag course, indicat- 
ing circumnutation. 

(6.) Trifolium resupi- 
natum (LegmninosaB, 
Fam. 75). — When we 
treat of the sleep of 
plants, we shall see that 
the stems in several 
Leguminous genera, for 
instance, those of Hedy- 
sarum, Mimosa, Meli- 
lotus, &c, which are not 
climbers, circumnutate 
in a conspicuous manner. 
We will here give only a single instance (Fig. 73), showing 
the circumnutation of the stem of a large plant of a clover, 
Trifolium resupinatum. In the course of 7 h. the stem changed 




Trifolium rcs'ipinatun ; circumnutation of 
stein, traced on vertical glass from 9.30 
a.m. to 4.30 p.m. Nov. 3rd. Tracing not. 
greatly rnagmfiel, reduced to half of 
original size. Plant feebly illuminated 
from above. 



Chap. IV 



CIRCUMNUTATIOX OF STEMS. 



205 



its course greatly eight times and completed three irregular 
circles or ellipses. It therefore circumnutated rapidly. Some 
of the lines run at right angles to one another. 



Fig. 74. 




(bus (hyboid) : circumnutation of stem, traced on horizontal glass, from 
4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magnified, re- 
duced to half of original size. Plant illuminated feebly from above. 

(7.) Rubus idceus (hybrid) (KosaceaB, Fam. 76). — As we hap- 



Fig. 75. 



pened to have a young plant, 11 inches 
in height and growing vigorously, 
which had been raised from a cross 
between the raspberry (Rubus idceus) 
and a North American Eubus, it was 
observed in the usual manner. During 
the morning of March 14th the stem 
almost completed a circle, and then 
moved far to the right. At 4 p.m. it 
reversed its course, and now a fresh 
tracing was begun, which was con- 
tinued during 40! h., and is given in 
Fig. 74. We here have well-marked 
circumnutation. 

(8.) JDeutzia gracilis (Saxifragese, 
Fam. 77). — A shoot on a bush about 
18 inches in height was observed. The 
bead changed its course greatly eleven 
times in the course of 10 h. 30 m. 
(Fig. 75), and there could be no 
doubt about the circumnutation of the 
stem. 

(9.) Fuchsia (greenhouse var,., with 
large flowers, probably a hybrid) (Ona- 
grariese, Fam. 100). — A young plant, 
15 inches in height, was observed during nearly 48 h. 




Deutzia gracilis ; circumnu- 
tation of stem, kept in 
darkness, traced on hori- 
zontal glass, from 8.30 
A.M. to 7 P.M.March 20th. 
Movement of bead origin- 
ally magnified about 20 
times, here reduced to 
half scale. 



The 



206 CIRCUMNUTATION OF STEMS. Chap. IV. 

accompanying figure (Fig. 76) gives the necessary particulars, 
and shows that the stem circumnutated, though rather 
Blowly. 



Fuchsia (garden var.) : circum nutation of stem, kept in darkness, traced CD 
horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of 
bead originally magnified about 40 times, here reduced to half scale. 

(10.) Cereus sptciocissimns (garden var., sometimes called 
Phyllocactus multiflorus) (Cactese, Fam. 109). — This plant 
which was growing vigorously from haviDg been removed a 
few clays before from the greenhouse to the hot- house, was 
observed with especial interest, as it seemed so little probable 
that the stem would circumnutate. The branches are flat, or 
fiabelliform ; but some of them are triangular in section, with 
the three sides hollowed out. A branch of this latter shape, 
9 inches in length and 1 i in diameter, was chosen for observa- 
tion, as less likely to circumnutate than a fiabelliform branch. 
The movement of the bead at the end of the glass filament, 
affixed to the summit of the branch, was traced (A, Fig. 77) 
from 9.23 a.m. to 4.30 p.m. on Nov. 23rd, during which time it 
changed its course greatly six times. On the 24th another 
tracing was made (see B), and the bead on this day changed its 
course oftener, making in 8 h. what may be considered as four 
ellipses, with their longer axes differently directed. The position 
of the stem and its commencing course on the following 
morning are likewise shown. There can be no doubt that this 
branch, though appearing quite rigid, circumnutated; but the 



Chap. IV 



CIBCUMNUTATION OF STEMS. 



207 



extreme amount of movement during the time was very small, 
probably rather less than the J^th of an inch. 

Fi K . 77. 



8T30\ 




^\4.°30'p.m. 



Jf2$a.m. 
A 



S°a.m..2& 




'Jereus speciocissimus ; circumnutation of stem, illuminated from above, 
traced on a horizontal glass, in A from 9 a.m. to 4.30 P.M. on Nov. 
23rd ; and in B from 8.30 a.m. on the 24th to 8 A.M. on the 25th. 
Movement of the bead in B magnified about 38 times. 



(11.) Hedtra "lelix (Araliacese, Fam. 114).— The stem is known 
to be apheliotropic, and several seedlings growing in a pot in 
the greenhouse became bent in the middle of the summer at 
right angles fiom the light. On Sept. 2nd some of these stems 
were tied up so as to stand vertically, and were placed before 
a north-east window; but to our surprise they were now 
decidedly heliotropic, for during 4 days they curved them- 
selves towards the light, and their course being traced on a 
horizontal glass, was strongly zigzag. During the 6 succeed- 
ing days they circumnutated over the same small space at a 
slow rate, but there could be no doubt about their circumnuta- 
tion. The plants were kept exactly in the same place before the 
window, and after an interval of 15 days the stems were 
again observed during 2 days and their movements traced, and 



208 



CIBCUMNUTATION OF STEMS. 



Chap. iy 



they were found to be still circumnutating, but on a yet smaller 
scale. 

(12.) Gazania ringens (Composite, Fam. 122).— The circum- 
nutation of the stem of a young plant, 7 inches in height, as 
measured to the tip of the highest leaf, was traced during 
33 h , and is shown in the accompanying figure (Fig. 78). Two 



Fig. 78. 



SM.nt.8ST 



P.ji.m.l 




Z'am.2Z nd ' 



'io° 3fp.m.2f 



Gazania rinjens : circunmutation of stem traced from 9 A.M. March 21st 
to 6 P.M. on 22nd; plant kept in darkness. Movement of bead at the 
close of the observations magnified 3-i times, here reduced to half the 
original scale. 

main lines may be observed running at nearly right angles to 
two other main lines; but these are interrupted by small 
loops. 

(13.) Azalea Indlca (Ericincse, Fam. 128). — A bush 21 inches 
in height was selected for observation, and the circunmutation 
of its leading shoot was traced during 26 h. 40 m , as shown 
in the following figure (Fig. 79). 

(14.) Plumbago C<<perisis (Plumbaginess, Fam. 134).— A small 
lateral branch which projected from a tall freely growing bush, 
at an angle of 35° above the horizon, was selected for obser- 
vation. For the first 11 h. it moved to a considerable distance 
in a nearly straight line to one side, owing probably to its 
having been previously deflected by the light whilst standing in 
the greenhouse. At 7.20 p.m. on March 7th a fresh tracing was 
begun and continued for the next 43 h. 40 m. (see Fig. 80). 
During the first 2 h. it followed nearly the same direction as 
before, and then changed it a little; during the night it 
moved at nearly right angles to its previous course. Next 



Chap. IV 



CIECUMNUTATION OF STEMS 



209 



day (8th) it zigzagged greatly, and on the 9th moved irregu- 
larly round and round a small circular space. By 3 p.m. on 
the 9th the figure had become so complicated that no more dots 
could be made ; but the shoot continued during the evening of 
the 9th, the whole of the 10th, and the morning of the 11th to 



Fig. 79. 



Fig. 80. 




Azalea Indica : circumnutatiou 
of stem, illuminated from 
abore, traced on horizontal 
glass, from 9.30 A.M. March 
9th to 12.10 P.M. on the 10th. 
But on the morning of the 
10th only four dots were 
made between 8.30 A.M. 
and 12.10 P.M., both hours 
included, so that the cireum- 
nutation is not fairly repre- 
sented in this part of the 
diagram. Movement of the 
bead here magnified about 
30 times. 




Plumbago Capensis : circumnu- 
tatiou of tip of a lateral 
branch, traced on horizontal 
glass, from 7.20 P.M. on 
March 7th to 3 p.m. on the 
9th. Movement of bead 
magnified 13 times. Plant, 
feebly illuminated from 
above. 



circumnutate over the same small space, which was only aboui 
the ^th of an inch ('97 mm.) in diameter. Although this 
branch circumnutated to a very small extent, yet it changed its 
course frequently. The movements ought to have been more 
magnified. 

(15.) Aloyua citriodora (Verbenacefle, Fain. 173). — The follow- 
ing figure (Fig. 81) gives the movements of a shoot during 



210 



CIKCUMNUTATION OF STEMS. 



Chap. IV. 



31 h. 40 m., and shows that it circumnutated. The bu*h was 
15 inches in height. 

Fig. 81. 




Aloy&ia ciiriodora : circumnutation of stem, traced from 8.20 A.M. on March 
22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement magnified 
about 40 times. 

(16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.) 
(Verbenaceae). — A shoot 8 inches in height had been laid hori- 
zontally, for the sake of observing its apogeotropism, and the 
terminal portion had grown vertically upwards for a length of 
1| inches. A glass filament, with a bead at the end, was fixed 



'a.m. 7*!* 



Fig. 82 . 




5°3(f]>.m.5? h 



6'50'a.m.6 l 

Verbena melindres: circumnutation of stem in d 
glass, from 5.30 p.m. on June 5th to 11 a.m. 
bead magnified 9 times. 



irkness, traced on vertical 
June 7th. Movement of 



upright to the tip, and its movements were traced during 
41 h. 30 m. on a vertical glass (Fig. 82). Under these circum- 
stances the lateral movements were chiefly shown ; but as the 
lines from side to side are not on the same level, the shoot 



Chap. IV. CIKCUMNUTATION OF STEMS. 211 

must have moved in a plane at right angles to that of the lateral 
movement, that is, it must have circumnutated. On the next day 
(6th) the shoot moved in the course of 16 h. four times to the right, 
and four times to the left; and this apparently represents the 
formation of four ellipses, so that each was completed in 4 h. 

(17.) Ceratophyttum demersum (Ceratophyilese, Fam. 220). — An 
interesting account of the movements of the stem of this water- 
plant has been published by M. E. Eodier,£. The movements are 
confined to the young internodes, becoming less and less lower 
down the stem ; and they are extraordinary from their amplitude. 
The stems sometimes moved through an angle of above 2.0° in 
6 h., and in one instance through 220° in 3 h. They generally 
bent from right to left in the morning, and in an opposite direc- 
tion in the afternoon ; but the movement was sometimes tempo- 
rarily reversed or quite arrested. It was not affected by light. 
It does not appear that M. Eodier made any diagram on a hori- 
zontal plane representing the actual course pursued by the 
apex, but he speaks of the " branches executing round their 
axes of growth a movement of torsion." From the particulars 
above given, and remembering in the case of twining plants and 
of tendrils, how difficult it is not to mistake their bending to all 
points of the compass for true torsion, we are led to believe that 
the stems of this Ceratophyllum circumnutate, probably in the 
shape of narrow ellipses, each completed in about 26 h. The 
following statement, however, seems to indicate something 
different from ordinary circumnutation, but we cannot fully 
understand it. M. Eodier says : " II est alors facile de voir que 
le mouvement de flexion se produit d'abord dans les merithalles 
superieurs, qu'il se propage ensuite, en s'amoindrissant du huut 
en bas; tandis quau contraire le mouvement de redressement 
commence par la par tie inferieure pour se terminer a la partie 
superieure qui, quelquefois, peu de temps avant de se relcver 
tout a fait, forme avec l'axe un angle tres aigu." 

(18 ) Coniferce. — Dr. Maxwell Masters states (' Journal Linn 
Soc.,' Dec. 2nd, 1879) that the leading shoots of many Coniferre 
during the season of their active growth exhibit very remark- 
able movements of revolving nutation, that is, they circumnu- 
tate. We may feel sure that the lateral shoots whilst growing 
would exhibit the same movement if carefully observed. 



* 'Coraptes Eendus,' April 30th. 1877. Also a second nuticte 
published separately in Bouidt-aux, Nov. 12th, 1877. 



212 CIBCUMNUTATION OF STEMS. Chap. IV 

(19.) Lillum auratum (Fam. Liliacese). — The- circummitation 

Fig. 83. 







6p.m]5Z-- 




XjgpOL 



8>am.U th 



Lillum aumtun: eireumnutation of a stem in darkness, traced on a horizontal 
glass, from 8 A.M. on March 14th to 8.35 A.M. on 16th. But it should 
be noted that our observations were interrupted between 6 P.M. on the 
14th and 12.15 P.M. on 15th, and the movements during this interval 
of 18 h. 15 m. are represented by a long broken line. Diagram reduced 
to half original scale. 

of the stem of a plant 2 1 inches in height is represented in the 
above figure (Fig. bS). 

Fig. 48. 




Oyperus rtlternifohus : eireumnutation of stem, illuminated from above, 
traced on horizontal glass,. from 9.45 A.M. March 9th to 9 P.M. on 10th 
The stem grew so rapidly whilst being observed, that it was not possible 
to estimate how much its movements were magnified in the tracing. 

(20.) Cyperus alternifoTius (Fam. Cyperacese.) — A glass 






Chap. IV. CIRCUMNUTATION OF STEMS. 213 

filament, with a bead at the end, was fixed across the summit 
of a young stem 10 inches in height, close beneath the crown of 
elongated leaves. On March 8th, between 12.20 and 7.20 p.m., 
the stem described an ellipse, open at one end. On the follow- 
ing day a new tracing was begun (Fig. 84), which plainly shows 
that the stem completed three irregular figures in the course of 
35 h. 15 m. 

Concluding Remarks on the Circumnutation of Stems.— 
Any one who will inspect the diagrams now given, and 
will bear in mind the widely separated position of the 
plants described in the series, — remembering that we 
have good grounds for the belief that the hypocotyls 
and epicotyls of all seedlings circumnutate, — not 
forgetting the number of plants distributed in the 
most distinct families which climb by a similar move- 
ment, — will probably admit that the growing stems 
of all plants, if carefully observed, would be found 
to circumnutate to a greater or less extent. When 
we treat of the sleep and other movements of plants, 
many other cases of circumnutating stems will be 
incidentally given. In looking at the diagrams, we 
should remember that the stems were always growing, 
so that in each case the circumnutating apex as it 
rose will have described a spire of some kind. The 
dots were made on the glasses generally at intervals 
of an hour, or hour and a half, and were then joined 
by straight lines. If they had been made at intervals 
of 2 or 3 minutes, the lines would have been more 
curvilinear, as in the case of the tracks left on the 
smoked glass-plates by the tips of the circumnutating 
radicles of seedling plants. The diagrams generally 
approach in form to a succession of more or less 
irregular ellipses or ovals, with their longer axes 
directed to different points of the compass during the 
same day or on succeeding days. The stems there- 



214 CIECUMNUTATION OF STOLONS. Chap. IV 

fore, sooner or later, bend to all sides; but after a 
stern has bent in any one direction, it commonly 
bends back at first in nearly, though not quite, the 
opposite direction ; and this gives the tendency to 
the formation of ellipses, which are generally narrow, 
but not so narrow as those described by stolons and 
leaves. On the other hand, the figures sometimes 
approach in shape to circles. Whatever the figure 
may be, the course pursued is often interrupted by 
zigzags, small triangles, loops, or ellipses. A stem 
may describe a single large ellipse one day, and 
two on the next. With different plants the com- 
plexity, rate, and amount of movement ..differs 
much. The stems, for instance, of Iberis and Azalea 
described only a single large ellipse in 24 h. ; 
whereas those of the Deutzia made four or five deep 
zigzags or narrow ellipses in 11 \ h., and those of the 
Trifolium three triangular or quadrilateral figures 
in 7 h. 

ClRCUMNUTATION OF STOLONS OR KuKNERS. 

Stolons consist of much elongated, flexible branches, 
which run along the surface of the ground and form 
roots at a distance from the parent-plant. They are 
therefore of the same homological nature as stems ; 
and the three following cases may be added to the 
twenty previously given cases. 

Frafjaria (cultivated garden var.) : Rosacea;. — A plant growirg 
m a pot had emitted a long stolon ; this was supported by a 
stick, so that it projected for the length of several inches hori- 
zontally. A glass filament bearing two minute triangles of 
paper was affixed to the terminal bud, which was a little up- 
turned ; and its movements were traced during 21 h., as shown 
in Fig. 85. In the course of the first 12 h. it moved twice up 
and twice down in somewhat zigzag lines, and no doubt tra- 
velled in the same manner during the night. On the followiug 



Chap. IV. CIRCUMNUTATION OF STOLONS. 



215 



morning after an interval of 20 h. the apex stood a little higher 
than it did at first, and this shows that the stolon had not been 

Fig. 85. 

9 6°45 l am.l9 1 ^ 



JO°pm 

Jff/lS'a.m 
48* 




3 r4S , amJ9 t 



Frag a ia : circumnutation of stolon, kept in darkness, traced on vertical 
glass, from 10.45 A.M. May 18th to 7.45 A.M. on 19th. 

acted on within this time by geotropism;* nor had its own 
weight caused it to bend downwards. 

On the following morning (19th) the glass filament was 
detached and refixed close behind the bud, as it appeared pos- 
sible that the circumnutation of the terminal bud and of the 
adjoining part of the stolon might be different. The movement 
was now traced during two consecutive days (Fig. 86). During 
the first day the filament travelled in the course of 14 h. 30 m. 
five times up and four times down, besides some lateral move- 
ment. On the 20th the course was even more complicated, and 
can hardly be followed in the figure ; but the filament moved in 
16 h. at least five times up and five times down, with very little 



* Dr. A. B. Frank states (• Die 

Naturliche wagerechte Richtung 

von Pflanzentheilen,' 1S70, p. 20) 

that the stolons of this plant are 

15 



acted on by geotropism, but only 
after a considerable interval o( 
time. 



216 



CIRCUMNUTATION OF STOLONS. Chap. IV 



lateral deflection. The first and last dots made on this second 
day, viz., at 7 a.m. and 11 p.m., were close together, showing 
that the stolon had not fallen or risen. Nevertheless, by com- 
paring its position on 
the morning of the 19th 
and 21st, it is obvious 
that the stolon had sunk ; 
and this may be attri- 
buted to slow bending 
down either from its own 
weight or from geotro- 
pism. 

During a part of the 20th 
an orthogonal tracing was 
made by applying a cube 
of wood to the vertical 
glass and bringing the 
apex of the stolon at suc- 
cessive periods into a line 
with one edge; a dot 
being made each time on 
the glass. This tracing 
therefore represented very 
nearly the actual amount 
of movement of the apex ; 
and in the course of 9 h. 
the distance of the ex- 
treme dots from one an- 
other was '45 inch. By 
the same method it was 
ascertained that the apex 
moved between 7 a.m. on 
8°a.m.2l? the 20th and 8 a.m. on the 
21st a distance of '82 inch. 
A younger and shorter 
stolon was supported so 
that it" projected at about 
movement was traced by the 
the first day the apex soon 
rose above the field of vision. By the next morning it had 
sunk, and the course pursued was now traced during 14 h. 
30 m. (Fig. 87). The amount of movement was almost the same, 




Fvagaria : circumnutation of the same stolon 
as in the last figure, observed in the same 
manner, and traced from 8 a.m. May 19th 
to 8 a.m. 21st. 



45° above the horizon, and its 
same orthogonal method. On 



Chap IV. CIKCUMNUTATION OF STOLONS. 217 

from side to side as up and down ; and differed in this respect 
remarkably from the movement in the previous cases. During 
the latter part of the day, viz., between 3 and 10.30 p.m., the 

Fig. 87. 




iCtam 



T20j).m 



Fragwia : circum nutation of another and younger stolon, traced from 
8 a.m. to 10.30 p.m. Figure reduced to one-half of original scale. 

actual distance travelled by the apex amounted to 1*15 inch; 
and in the course of the whole day to at least 2*67 inch. This 
is an amount of movement almost comparable with that of 
some climbing plants. The same stolon was observed on the 
following day, and now it moved in a somewhat less complex 
manner, in a plane not far from vertical. The extreme amount 
of actual movement was 1*55 inch in one direction, and "6 inch 
in another direction at right angles. During neither of these 
days did the stolon bend downwards through geotropism or its 
own weight. 

Four stolons still attached to the plant were laid on damp 
sand in the back of a room, with their tips facing the north-east 
windows. They were thus placed because De Tries says * that 
they are apheliotropic when exposed to the light of the sun ; but 
we could not perceive any effect from the above feeble degree of 
illumination. We may add that on another occasion, late in the 
summer, some stolons, placed upright before a south-west window 



Arbeiten Bot. Inst., Wiirzburg,' 1872, p. 434. 



218 CIKCUMNUTATION OF STOLONS. Chap IV. 

on a cloudy day, became distinctly curved towards the light, and 
were therefore heliotropic. Close in front of the tips of the 
prostrate stolons, a crowd of very thin sticks and the dried 
haulms of grasses were driven into the sand, to represent the 
crowded stems of surrounding plants in a state of nature. This 
was done for the sake of observing how the growing stolons 
would pass through them. They did so easily in the course of 
6 days, and their circumnutation apparently facilitated their 
passage. When the tips encountered sticks so close together 
that they could not pass between them, they rose up and passed 
over them. The sticks and haulms were removed after the 
passage of the four stolons, two of which were found to have 
assumed a permanently sinuous shape, and two were stiU 
straight. But to this subject we shall recur under Saxifraga. 

Saxifraga sarmentosa (Saxifrages). — A plant in a suspended 
pot had emitted long branched stolons, which depended like 

Fig. 88. 



I. 







Saxifraga sarmentosa: circumnutation of an inclined stolon, traoed in 
darkness on a horizontal glass, from 7.45 A.M. April 18th to 9 A.M. on 
9th. Movement of end of stolon magnified 2-2 times. 

threads on all sides. Two were tied up so as to stand vertically, 
and their upper ends became gradually bent downwards, but so 
slowly in the course of several days, that the bending was pro- 
bably due to their weight and not to geotropism. A glass fila- 
ment with little triangles of paper was fixed to the end of one of 
these stolons, which was 17J inches in length, and had already 
become much bent down, but still projected at a considerable 
angle above the horizon. It moved only slightly three times 
from side to side and then upwards; on the following day 



Chap. IV. CIRCUMNUTATION OF STOLONS. 219 

the movement was even less. As this stolon was so long we 
thought that its growth was nearly completed, so we tried 
another which was thicker and shorter, viz., 10} inches in length. 
It moved greatly, chiefly upwards, and" changed its course five 
times in the course of the day. During the night it curved sc 
much upwards in opposition to gravity, that the movement 
could no longer be traced on the vertical glass, and a horizontal 
one had to be used. The movement was followed during the 
next 25 h., as shown in Fig' 88. Three irregular ellipses, ivith 
their longer axes somewhat differently directed, were almost 
completed in the first 15 h. The extreme actual amount of 
movement of the tip during the '25 h. was '75 inch. 

Several stolons were laid on a flat surface of damp sand, in the 
same manner as with those of the strawberry. The friction of 
the sand did not interfere with their circumnutation ; nor could 
we detect any evidence of their being sensitive to contact. In 
order to see how in a state of nature they would act, when 
encountering a stone or other obstacle on the ground, short 
pieces of smoked glass, an inch in height, were stuck upright 
into the sand in front of two thin lateral branches. Their tips 
scratched the smoked surface in various directions ; one made 
three upward and two downward lines, besides a nearly hori- 
zontal one; the other curled quite away from the glass; but 
ultimately both surmounted the glass and pursued their original 
course. The apex of a third thick stolon swept up the glass in a 
curved line, recoiled and again came into contact with it ; it then 
moved to the right, and after ascending, descended vertically ; 
ultimately it passed round one end of the glass instead of over it. 

Many long pins were next driven rather close together into 
the sand, so as to form a crowd in front of the same two thin 
lateral branches; but these easily wound their way through 
the crowd. A thick stolon was much delayed in its passage ; 
at one place it was forced to turn at right angles to its former 
course; at another place it conld not pass through the pins, 
and the hinder part became bowed ; it then curved upwards 
and passed through an opening between the upper part of some 
pins which happened to diverge ; it then descended and finally 
emerged through the crowd. This stolon was rendered perma- 
nently sinuous to a slight degree, and was thicker where sinuous 
than elsewhere, apparently from its longitudinal growth having 
been checked. 

Cotyledon umbilicus (Crassulaceae).— A plant growing in a pan 



220 



CIRCUMNUTATION OF STOLONS. Chap. IV 



of damp moss had emitted 2 stolons, 22 and 20 inches in length 
One of these was supported, so that a length of 4£ inches pro* 
iected in a straight and horizontal line, and the movement 
of the apex was traced. The first dot was made at 9.10 a.m. 



Fig. 89. 



fJVl5 , a.nb.2S f !' 




f,G°10 1 a.m.37( h 



ijfa.7ti.27 t ? 1 



5"SC?jMn.26 e £ 



10°35'p.m.2&h 



Cotyledon umbilicus: circumnutation of stolon, traced from 11.15 A.M 
Aug. 25th to 11 a.m. 27th. Plant illuminated from above. TV 
terminal internocle was *25 inch in length, the penultimate 2 -25, ana 
the third 3 inches in length. Apex of stolon stood at a distance of 
5*75 inches from the vertical glass ; but it was not possible to ascertain 
how much the tracing was magnified, as it was not known how great 
a length of the internode circumnutated. 



the terminal portion soon began to bend downwards and con- 
tinned to do so nntil noon. Therefore a straight line, very 
nearly as long as the whole figure here given (Fig. 89), was first 
traced on the glass ; but the upper part of this line has not been 
copied in the diagram. The curvature occurred in the middle 



Chap. IV. 



CIKCUMNUTATION OF STOLONS. 



221 



of the penultimate internode; and its chief seat was at the 
distance of lj inch from the apex; it appeared due to the 
weight of the terminal portion, acting on the more flexible 
part of the internode, and not to geotropism. The apex after 
thus sinking down from 9.10 a.m. to noon, moved a little to the 
left; it then rose up and circumnutated in a nearly vertical 
plane until 10.35 p.m. On the following day (26th) it was ob- 

Fig. 90. 

J°U' a.m.25 l b 




W3£part.25f!\ o , 



6°m'a.m.2Q$ 




^20 r jp.m.26 t ! 



Cotyledon umbilicus: circumnutation and downward movement of another 
ttolon, traced on vertical glass, from 9.11 a.m. Aug. 25th to 11 A.m. 27th. 
Apex close to glass, so that figure but little magnified, and here reduced 
to two-thirds of original size. 

served from 6.40 a.m. to 5.20 p.m., and within this time it moved 
twice up and twice down. On the morning of the 27th the apex 
stood as high as it did at 11.30 a.m. on the 25th. • Nor did it 
sink down during the 28th, but continued to circumnutate about 
the same place. 
Another stolon, which resembled the last in almost every 



222 CIRCUMNUTATION OF STOLONS. Chap. IV. 

respect, was observed during the same two days, but only two 
inches of the terminal portion was allowed to project freely and 
horizontally. On the 25th it continued from 9.10 a.m. to 1.30 r.M. 
to bend straight downwards, apparently owing to its weight 
(Fig. 90); but after this hour until 10.35 p.m. it zigzagged. 
This fact deserves notice, for we here probably see the combined 
effects of the bending down from weight and of circumnutation. 
The stolon, however, did not circumnutate when it first began 
to bend down, as may be observed in the present diagram, and 
as was still more evident in the last case, when a longer portion 
of the stolon was left unsupported. On the following day 
(26th) the stolon moved twice up and twice down, but still con- 
tinued to fall ; in the evening and during the night it travelled 
from some unknown cause in an oblique direction. 

We see from these three cases that stolons or 
runners circumnutate in a very complex manner. The 
lines generally extend in a vertical plane, and this 
may probably be attributed to the effect of the weight 
of the unsupported end of the stolon ; but there is 
always some, and occasionally a considerable, amount 
of lateral movement. The circumnutation is so great 
in amplitude that it may almost be compared with 
that of climbing plants. That the stolons are thus 
aided in passing over obstacles and in winding between 
the stems of the surrounding plants, the observations 
above given render almost certain. If they had not 
circumnutated, their tips would have been liable to 
have been doubled up, as often as they met with 
obstacles in their path ; but as it is, they easily avoid 
them. This must be a considerable advantage to the 
plant in spreading from its parent-stock ; but we are 
far from supposing that the power has been gained 
by the stolons for this purpose, for circumnutation 
seems to be of universal occurrence with all growing 
parts; but it is not improbable that the amplitude 
of the movement may have been specially increased 
for this purpose. 



Chai IV. CIECUMNUTATION OF FLO WEE -STEMS. 223 



ClRCUMNUTATION OF FlOWEE-STEMS. 

We did not think it necessary to make any special 
observations on the circumnutation of flower-sterns, 
these being axial in their nature, like stems or stolons ; 
but some were incidentally made whilst attending 
to other subjects, and these we will here briefly give. 
A few observations have also been made by other 
botanists. These taken together suffice to render it 
probable that all peduncles and sub-peduncles cir- 
cumnutate whilst growing. 

Oxalis caraosa.-— The peduncle which springs from the thick 
and woody stem of this plant bears three or four suo-peduncles. 

Fig. 91. 




Oxalis camosa : flower-stem, feebly illuminated from above, its circumnuta 
tion traced from 9 A.M. April 13th to 9 A.M. 15th. Summit of flowei 
8 inches beneath the horizontal glass. Movement probably magnified 
about 6 times. 

A filament with little triangles of paper was fixed within the 
calyx of a flower which stood upright. Its movements were 
observed for 48 h. ; during the first half of this time the flower 
was fully expanded, and during the second half withered. The 
figure here given (Fig. 91) represents 8 or 9 ellipses. Although 
the main peduncle circumnutated, and described one large and 



224 CIRCUMNUTATION OF FLOWER-STEMS Chat, I\ 

two smaller ellipses in the course of 24 h., yet the chief seat oi 
movement lies in the sub-peduncles, which ultimately bend 
vertically downwards, as will be described in a future chapter. 
The peduncles of Oxalis acetosella likewise bend downwards, and 
afterwards, when the pods are nearly mature, upwards ; and this 
is effected by a circumnutating movement. 

It may be seen in the above figure that the flower-stem of 
O.carnosa circumnutated during two days about the same spot. 
On the other hand, the flower-stem of (J. sensitiva undergoes a 
strongly marked, daily, periodical change of position, when kept 
at a proper temperature. In the middle of the day it stands 
vertically up, or at a high angle ; in the afternoon it sinks, and 
in the evening projects horizontally, or almost horizontally, 
rising again during the night. This movement continues from 
the period when the no wers are in bud to when, as we believe, 
the pods are mature : and it ought perhaps to have been included 
amongst the so-called sleep-movements of plants. A tracing 
was not made, but the angles were measured at successive periods 
during one whole day; and these showed that the movement 
was not continuous, but that the peduncle oscillated up and 
down. We may therefore conclude that it circumnutated. At 
the base of the peduncle there is a mass of small cells, forming 
a well- developed pulvinus, which is exteriorly coloured purple 
and hairy. In no other genus, as far as we kno w, is the peduncle 
furnished with a pulvinus. The peduncle of 0. Ortegesii behaved 
differently from that of 0. sensitiva, for it stood at a less angle 
above the horizon in the middle of the day, than in the morning 
or evening. By 10.20 p.m. it had risen greatly. During the 
middle of the day it oscillated much up and down. 

Trifolium sublerraneum. — A filament was fixed vertically to 
the uppermost part of the peduncle of a yourjg and upright 
flower-head (the stem of the plant having been secured to a 
stick); and its movements were traced during 36 h. Within 
this time it described (see Fig. 92) a figure which represents four 
ellipses; but during the latter part of the time the peduncle 
began to bend downwards, and after 10.30 p.m. on the 24th it 
curved so rapidly down, that by 6.45 a.m. on the 25th it stood 
only 19° above the horizon. It went on circumnu fating in nearly 
the same position for two dajs. Even after the flower-heads 
have buried themselves in the ground they continue, as will 
hereafter be shown, to circumnutate. It will also be seen in the 
uext chapter that the sub-peduncles of the separate flowers of 



Chap. IV. CIKCUMNUTATION OF FLOWER-STEMS. 225 



Trifolium repens circumnutate in a complicated course during 
Beveral days. I may add that the gynophore of Arnolds hypogaa, 



Fig. 92. 




'40'a.m 23*5* 



\Hf.30:p,m.24^ 



Trifolium subterraneum : main flower-peduncle, illuminated from above, 
circumnutation traced on horizontal glass, from 8.40 A.M. July 23rd 
to 10.30 P.M. 24th. 

which looks exactly like a peduncle, circumnutates whilst growing 
vertically downwards, in order to bury the young pod in the 
ground. 

The movements of the flowers of Cyclamen Persicum were not 
observed ; but the peduncle, whilst the pod is forming, increases 
much in length, and bows itself down by a circumnutating 
movement. A young peduncle of Maurandia semperflorens, 
li inch in length, was carefully observed during a whole day, 
and it made 4 \ narrow, vertical, irregular and short ellipses, 
each at an average rate of about 2 h. 25 m. An adjoining 
peduncle described during the same time similar, though fewer, 
.* According to Sachs f the flower-stems, whilst growing, 



* ' The Movements and Habits 
of Climbing Plants,' 2nd edit., 



1875, p. 68. 
t ' Text-Book of Botany,' 1875, 



226 CIRCUMNUTATION OF LEAVES. Chap. IV. 

of many plants, for instance, those of Brassica napus, revolve or 
circumnutate ; those of Allium porrum bend from side to side, 
and, if this movement had been traced on a horizontal glass, 
no doubt ellipses would have been formed. Fritz Miiller has 
described * the spontaneous revolving movements of the flower- 
stems of an Alisma, which he compares with those of a climbing 
plant. 

We made no observations on the movements of the different 
parts of flowers. Morren, however, has observed f in the 
stamens of Sparmannia and Cereus a " frenJssement spontane," 
which, it may be suspected, is a circumnutating movement. 
The circumnutation of the gynostemium of Stylidium, as de- 
scribed by Gad,J is highly remarkable, and apparently aids in 
the fertilisation of the flowers. The gynostemium, whilst spon- 
taneously moving, comes into contact with the viscid labellum, 
to which it adheres, until freed by the increasing tension of the 
parts or by being touched. 

We have now seen that the flower-stems of plants 
belonging to such widely different families as the 
Cruciferse, Oxalidge, LeguminosaB, Primulaceae, Scro- 
phularinese, Alismacese, and Liliacese, circumnutate ; 
and that there are indications of this movement in 
many other families. With these facts before us, 
bearing also in mind that the tendrils of not a few 
plants consist of modified peduncles, we may admit 
without much doubt that all growing flower-stems 
circumnutate. 

Circumnutation of Leaves : Dicotyledons. 

Several distinguished botanists, Hofmeister, Sachs, 
Pfeffer, De Vries, Batalin, Millardet, &c, have ob- 



p. 766. Linnreus and Treviranus plies circumnutation. 

(according to Pfeffer, ' Die Pe- * ' Jenaische Zeitsch./ B. v. 

riodischen Bewegungen/ &c.„ p. p. 133. 

162) state that the flower-stalks f ' N. Mem. de l'Acad. E. de 

of many plants occupy different Bruxelles,' torn. xiv. 1841, p. 3. 

positions by night and day, and % ' Sitzungbericht des bot. Ve- 

we shall s< e in the chapter on reins der P. Brandenburg,' xxi 

the Sleep of Plants that this im- p. 84. 



Chap IV. 



DICOTYLEDONS. 



227 






served, and some of them with the greatest care, the 
periodical movements of leaves ; bat their attention 
has been chiefly, though not exclusively, directed to 
those which move largely and are commonly said to 
sleep at night. From considerations hereafter to be 
given, plants of this nature are here excluded, and 
will be treated of separately. As we wished to ascer- 
tain whether all young and growing leaves circumnu- 
tated, we thought that it would be sufficient if we 
observed between 30 and 40 genera, widely distributed 
throughout the vegetable series, selecting some un- 
usual forms and others on woody plants. All the 
plants were healthy and grew in pots. They were 
illuminated from above, but the light perhaps was not 
always sufficiently bright, as many of them were ob- 
served under a skylight of ground-glass. Except in a 
few specified cases, a fine glass filament with two minute 
triangles of paper was fixed to the leaves, and their 
movements were traced on a 
vertical glass (when not stated 
to the contrary) in the manner 
already described. I may repeat 
that the broken lines represent 
the nocturnal course. The stem 
was always secured to a stick, 
close to the base of the leaf 
under observation. The ar- 
rangement of the species, with 
the number of the Family ap- 
pended, is the same as in the 
case of stems. 



Fig. 93. 




Sarracenia purpurea : circum- 
nutation of young pitcher, 
traced from 8 A.M. July 3rd 
to 10.15 a.m. 4th. Temp. 
17°-1 8° C. Apex of pitcher 
20 inches from glass, so 
movement greatly mag- 
nified. 



(1.) Sarracenia purpurea (Sarra- 
ceneae, Fam. 11).— A young leaf, or 

pitcher, 8^ inches in height, with the bladder swollen, but with 
the hood not as yet open, had a filament fixed transversely 



228 



CIKCUMNUTATION OF LEAVES. 



Chap. IV. 



across its apex ; it was observed for 4S h., and during the whole 
of this time it circumnutated in a nearly similar manner, but 
to a very small extent. The tracing given (Fig. 93) relates 
only to the movements during the first 26 h. 

(2.) Glaucium luteum (Papave- 
racese, Fam. 12).— A young plant, 
bearing only 8 leaves, had a fila- 
ment attached to the youngest leaf 
but one, which was 3 inches in 
length, including the petiole. The 
circumnutating movement was 
traced during 47 h. On both days 
the leaf descended from before 7 a.m. 
until about 11 a.m., and then 
ascended slightly during the rest 
of the day and the early part of 
the night. During the latter part 
of the night it fell greatly. It did 
not ascend so much during the 
second as during the first day, and 
it descended considerably lower on 
the second night than on the first. 
This difference was probably due 
to the illumination from above 
having been insufficient during the 
two days of observation. Its course 
during the two days is shown in 
Fig. 94. 

(3.) Crambe maritima (Cruciferse, 
Fam. 14). — A leaf 9s inches in length 
on a plant not growing vigorously 
was first observed. Its apex was 
in constant movement, but this 
could hardly be traced, from being 
so small in extent. The apex, how- 
ever, certainly changed its course at 
least 6 times in the course of 14 h. 
A more vigorous young plant, bear- 
ing only 4 leaves, was then selected, 
and a filament was affixed to the 
midrib of the third leaf from the base, which, with the petiole, was 
6 inches in length. The leaf stood up almost vertically, but the^tip 




Glaucium luteum : circumnuta- 
tion of young leaf, traced 
from 9.30 a.m. June 14th 
to 8.30 A.M. 16th. Tracing 
not much magnified, as apex 
of leaf stood only 5£ inches 
from the glass. 



Chap. IT. 



DICOTYLEDONS. 



229 



was deflected, so that the filament projected almost horizontally, 

and its movements were traced during 48 h. on a vertical glass, 

as shown in the accompanying figure (Fig. 95). We here plainly 

see that the leaf was con- • 

Fig. 95. 

6°40'a.m.25 i h 



S£p.m.83& 



M>°20'2.m.24& 



7'. 50' a.m. 23^ 



tinually circumnutatmg ; 
but the proper periodicity 
of its movements was dis- 
turbed by its being only 
dimly illuminated from 
above through a double 
skylight. We infer that 
this was the case, because 
two leaves on plants grow- 
ing out of doors, had their 
angles above the horizon 
measured in the middle 
of the day and at 9 to 
about 10 p.m. on succes- 
sive nights, and they 
were found at this latter 
hour to have risen by an 
average angle of 9° above 
their mid-day position : 
on the following morning 
they fell to their former 
position. Now it may be 
observed in the diagram 
that the leaf rose during 
the second night, so that 
it stood at 6.40 a.m. higher 
than at 10.20 p.m. on the 
preceding night ; and this 
may be attributed to the 
leaf adjusting itself to the 
dim light, coming exclu- 
sively from above. 

(4.) Brassica oleracea (Cruciferae).— Hofmeister and Batalin * 
state that the leaves of the cabbage rise at night, and fall by 
day. We covered a young plant, bearing 8 leaves, under a large 
bell-glass, placing it in the same position with respect to the 




£3fam.84$\ 



3°j>,m2<tfb 



Crambe maritima : circumnutation of leaf, 
disturbed by being insufficiently illumi- 
nated from above, traced from 7.50 A.M. 
June 23rd to 8 A.M. 25th. Apex of leaf 
15J inches from the vertical glass, so that 
the tracing was much magnified, but is 
here reduced to one-fourth of original scale. 



* ' Flora,' 1873, p. 437 



230 CIECUMNUTATION OF LEAVES. Chap. rV 

light in which it had long remained, and a filament was fixed 
at the distance of '4 of an inch from the apex of a young leaf 
nearly 4 inches in length. Its movements were then traced 
during three days, but the tracing is not worth giving. The 
leaf fell during the whole morning, and rose in the evening and 
during the early part of the night. The ascending and descend- 
ing lines did not coincide, so that an irregular ellipse was formed 
each 24 h. The basal part of the midrib did not move, as was 
ascertained by measuring at successive periods the angle which 
it formed with the horizon, so that the movement was confined 
to the terminal portion of the leaf, which moved through an 
angle of 11° in the course of 24 h., and the distance travelled by 
the apex, up and down, was between ■ 8 and • 9 of an inch. 

In order to ascertain the effect of darkness, a filament was 
fixed to a leaf 5^ inches in length, borne by a plant which after 
forming a head had produced a stem. The leaf was inclined 
44° above the horizon, and its movements were traced on a 
vertical glass every hour by the aid of a taper. During the 
first day the leaf rose from 8 a.m. to 10.40 p.m. in a slightly 
zigzag course, the actual distance travelled by the apex being 
• 67 of an inch. During the night the leaf fell, whereas it ought 
to have risen ; and by 7 a.m. on the following morning it had 
fallen ■ 23 of an inch, and it continued falling until 9.40 a.m. It 
then rose until 10.50 p.m., but the rise was interrupted by one 
considerable oscillation, that is, by a fall and re-ascent. During 
the second night it again fell, but only to a very short distance, 
and on the following morning re-ascended to a very short 
distance. Thus the normal course of the leaf was greatly 
disturbed, or rather completely inverted, by the absence of 
light ; and the movements were likewise greatly diminished in 
amplitude. 

We may add that, according to Mr. A. Stephen Wilson,* the 
young leaves of the Swedish turnip, which is a hybrid between 
B. oleracea and rapa, draw together in the evening so much 
" that the horizontal breadth diminishes about 30 per cent, of 
the daylight breadth." Therefore the leaves must rise con- 
siderably at night. 

(5.) Dianihus caryophyllus (Caryophyllese, Fain. 26). — The 



* 'Trans. Bot. Soc. Edinburgh,' see Darwin, 'Animals and Plants 
vol. xiii. p. 32. With resptct to under Domestication,' 2nd edit. 
the origin of the Swedish turnip, vol. i. p. 344. ■ 



Chap. IV. 



DICOTYLEDONS. 



231 



terminal shoot of a young plant, growing very vigorously, was 
selected for observation. The young leaves at first stand up 
vertically and close together, but they soon bend outwards and 
downwards, so as to become horizontal, and often at the same 
time a little to one side. A filament was fixed to the tip of a 
young leaf whilst still highly inclined, and the first dot was 
made on the vertical glass at 8.30 a.m. June 13th, but it curved 
downwards so quickly that by 6.40 a.m. on the following 
morning it stood only a little above the horizon. In Fig. 96 



96 



flO'-j 



l5'p.mJ3$ 




JO°35 , p.mie^ 




I / 

§ {6°4o'cum.U& 



I(f45p.m.l4*tp>* 



Dianthus caryophyllus : circumnutation of young leaf, traced from 10.15 
P.M. June 13th to 10.35 p.m. 16th. Apex of leaf stood, at the close of 
our observations, 8f inches from the vertical glass, so tracing not 
greatly magnified. The leaf was h\ inches long. Temp. 15^°-17£° C. 

the long, slightly zigzag line representing this rapid downward 
course, which was somewhat inclined to the left, is not given ; 
but the figure shows the highly tortuous and zigzag course, 
together with some loops, pursued during the next 2| days. 
As ths leaf continued to move all the time to the left, it is 
evident that the zigzag line represents many circumnutations. 

(6.) Camellia Japonica (Camelliacese, Fam. 32). — A youngish 
leaf, which together with its petiole was 21 inches in length and 
which arose from a side branch on a tall bush, had a filament 
attached to its apex. This leaf sloped downwards at an angle 
of 40° beneath the horizon. As it was thick and rigid, and its 
16 



232 



CIKCUMNUTATION OF LEAVES. 



Chap. 11 



petiole very short; 
Ficr. 97. 




much movement could not be expected 
Nevertheless, the apes changed its course 
completely seven times in the course of 
lis h., but moved to only a very small 
distance. On the next day the movement 
of the apex was traced during 26 h. 20 m. 
(as shown in Fig. 97), and was nearly ol 
the same nature, but rather less complex. 
The movement seems to be periodical, for 
on both days the leaf circumnutated in the 
forenoon, fell in the afternoon (on the first 
day until between 3 and 4 p.m., and on the 
second day until 6 p.m.), and then rose, 
falling again during the night or early 
morning. 
In the chapter on the Sleep of Plants 
we shall see that the leaves in several Malvaceous genera sink 

Fig. 98. 
°. 30'ajn.uth 



Camellia Japonica : cir- 
cumnutation of leaf, 
traced from 6.40 
A.M. June 14th to 
6.50 A.M. loth. 
Apex of leaf 12 
inches from the ver- 
tical crlass, so figure 
considerably mag- 
nified. Temp. 16°- 
16§° C. 



B*50'a.mJ8 f b 




\m a .45\p. 



(S'.SO'a.m.l^ 



10 .25'p.m.ioty 

Pelargonium zonnle : circumnutation and downward movement of young 
leaf, traced from 9.30 a.m. June 14th to 6.30 p.m. 16th. Apex of leaf 
9| inches from the vertical glass, so figure moderately magnified. 
Temp. 15°-16£° C. 

at night; and as they often do not then occupy a vertical 
position, especially if they have not been well illuminated during 



Chap. IV. 



DICOTYLEDONS. 



233 



the day, it is doubtful whether some of these cases ought not 
to have been included in the present chapter. 

(7.) Pelargonium zonule (Geraniacea), Fam. 47). — A young 
leaf, lj inch in breadth, with its petiole 1 inch long, borne on 
a young plant, was observed in the usual manner during 61 h. ; 
and its course is shown in the preceding figure (Fig. 98), 
During the first day and night the leaf moved downwards, but 
circumnutated between 10 a.m. and 4.30 p.m. On the second 
day it sank and rose again, but between 10 a.m. and 6 p.m. it 
circumnutated on an extremely small scale. On the third day 
the circumnutation was more plainly marked. 

(8.) Cissus discolor (Ampelideae, Fam. 67). — A leaf, not nearly 



Fig. 99. 



full-grown, the third from the apex of 
a shoot on a cut-down plant, was 
observed during 31 h. 30 m. (see Fig. 
99). The day was cold (15°-16° C), 
and if the plant had been observed in 
the hot-house, the circumnutation, 
though plain enough as it was, would 
probably have been far more con- 
spicuous. 

(9.) Vicia fdba (Leguminosas, Fam. 
75). — A young leaf, 3'1 inches in 
length, measured from base of petiole to 
end of leaflets, had a filament affixed 
to the midrib of one of the two ter- 
minal leaflets, and its movements were 
traced during 5H h. The filament fell 
all morning (July 2nd) till 3 p.m., and 
then rose greatly till 10.35 p.m. ; but 
the rise this day was so great, com- 
pared with that which subsequently 
occurred, that it was probably due in 
part to the plant being illuminated 
from above. The latter part of the course on July 2nd is alone 
given in the following figure (Fig. 100). On the next day 
(July 3rd) the leaf again fell in the morning, then circumnu- 
tated in a conspicuous manner, and rose till late at night ; but 
the movement was not traced after 7.15 p.m., as by that time the 
filament pointed towards the upper edge of the glass. During 
the latter part of the night or early morning it again fell in the 
same manner as before. 




Cissus disccfor ; circumnu- 
tation of leaf, traced 
from 10.35 A.M. May 
28th to 6 p.m. 29th. 
Apex of leaf 8| inches 
from the vertical glass. 



234 



CIBCUMXUTATION OF LEAVES. 



Chap. IV 



As tlio evening rise and the early morning fall were unusually 
large, the angle of the petiole above the horizon was measured 
at the two periods, and the leaf was found to have risen 10 e 

Fig. 100. 



6*48 f a.m.3Z* 




6Z45'am.4fa 



".15' a.m. 4$ 



Vicia f"bi: circumnutation of leaf, traced from 7.15 p.m. July 2nd tc 
10.15 A.M. 4th. Apex of the two terminal leaflets 1\ inches from the 
vertical glass. Figure here reduced to two-thirds of original scale. 
Temp. 17°-18° C. 

between 12.20 p.m. and 10.45 p.m., and to have fallen 23° 30 

between the latter hour and 10.20 a.m. on the following morning. 

The main petiole was now secured to a stick close to the base 



Chap. IV. 



DICOTYLEDONS. 



235 



of tlie two terminal leaflets, which were 1 4 inch in length ; and 
the movements of one of them were traced during 48 h. (see 
Fig. 101). 'The course pursued is closely analogous to that of 
the whole leaf. The zigzag line between 8.30 a.m. and 3.30 p.m. 
on the second day represents 5 very small ellipses, with their 



Fig. 101. 



JO°J5'jp.m/. 




10 a 30'a.m.6 e ! i 



lO^O'a.m. 



Vicia faba: circumnutation of one of the two terminal leaflets, the main 
petiole having been secured, traced from 10.40 A.M. July 4th to 10.30 A.M> 
6th. Apex of leaflet 6| inches from the vertical glass. Tracing here 
reduced to one-half of original scale. Temp. 16°-18° C. 

longer axes differently directed. From these observations it 
follows that both the whole leaf and the terminal leaflets undergo 
a well-marked daily periodical movement, rising in the evening 
and falling during the latter part of the night or early morning; 
whilst in the middle of the day they generally circumnutate 
round the same small space. 



236 



CIBCUMNUTATION OF LEAVES. 



Cuaf. rv 




(10.) Acacia retinoides (Leguminosae).— The movement of a 
young phyllode, 2$ inches in length, and inclined at a consider- 
able angle above the horizon, was traced 
during 45 h. 30 m. ; but in the figure here 
given (Fig. 102), its circumnutation is shown 
during only 21 h. 30 m. During part of 
this time (viz., 14 h. 30 m.) the phyllode 
described a figure re- 
presenting 5 or 6 
small ellipses. The 
actual amount of 
movement in a ver- 
tical direction was *3 
inch. The phyllode 
rose considerably be- 
tween 1.30 p.m. and 
4 p.m., but there was 
no evidence on either 
day of a regular pe- 
riodic movement. 

(11.) Lupinus spe- 
ciosus (Leguminosee). 
— Plants were raised 

from seed purchased under this name. 

This is one of the species in this large 

genus, the leaves of which do not sleep 

at night. The petioles rise direct from 

the ground, and are from 5 to 7 inches 

in length. A filament was fixed to the 

midrib of one of the longer leaflets, and 

the movement of the whole leaf was traced, 

as shown in Tig. 103. In the course of 

6 h. 30 m. the filament went four times up 

and three times down. A new tracing 

was then begun (not here given), and 

during 12 1 h. the leaf moved eight times 

up and seven times down; so that it 

described 1\ ellipses in this time, and 

this is an extraordinary rate of movement. 

The summit of the petiole was then secured 

to a stick, and the separate leaflets were found to be continually 

circumnutating. 



Acacia retinoides : cir- 
cumnutation of a 
young phyllode, 
traced from 10.45 
A.M. July 18th to 
8.15 a.m. 19th. 
Apex of phyllode 9 
inches from the 
vertical glass; temp. 
16£°-17|° C 




Lvpinus specioaus: cir- 
cumnutation of leaf 
traced on vertical 
glass, from 10.15 a.m. 
to 5.45 p.m.; i.e., 
during 6 h. 30 ui. 



Chap. IV. 



DICOTYLEDONS. 



237 



(12.) Echeveria stohnifera (Crassulaceae, Fam. 84). — The older 
leaves of this plant are so thick and fleshy, and the young ones 
so short and broad, that it seemed 
very improbable that any circum- 
nutation could be detected. A fila- 
ment was fixed to a young upwardly 
inclined leaf, "75 inch in length and 
•28 in breadth, which stood on the 
outside of a terminal rosette of leaves, 
produced by a plant growing very 
vigorously. Its movement was traced 
during 3 days, as here shown (Fig. 
104). The course was chiefly in an 
upward direction, and this may be 
attributed to the elongation of the 
leaf through growth ; but we see that 
the lines are strongly zigzag, and that 
occasionally there was distinct cir- 
cumnutation, though on a very small 
scale. 

(13.) Bryophyllum (vel Culanchct) 
calycinum (Crassulaceae). — Duval- 
Jouve ('Bull. Soc. Bot. de France,' 
Feb. 14th, 1868) measured the dis- 
tance between the tips of the upper 
pair of leaves on this plant, with the result shown in the following 
Table. It should be noted that the measurements on Dec. 2nd 
were made on a different pair of leaves : — 




Echeveria stohnifera : circum- 
nutation of leaf, traced 
from 8.20 A.M. June 25th 
to 8.45 A M. 28th. Apex 
of leaf 121 inches from the 
glass, so that the movement 
was much magnified; temp. 
23°-24J C. 





8 A.M. 


2 r.M. 


7 P.M. 


Nov. 16 . 


. 15 mm. 


. 25 mm. 


. • (?) 


„ 19 • 


. . 48 „ . 


. . 60 „ . 


. 48 mm 


Dec. 2 . 


. . 22 „ . 


. . 43 „ . 


. . 28 „ 



We see from this Table that the leaves stood considerably 
further apart at 2 p.m. than at either 8 am. or 7 p.m. ; and this 
shows that they rise a little in the evening and fall or open 
in the forenoon. 

(14.) Drosera rotundlfolia (Droseracea3, Fam. 85). — The move- 
ments of a young leaf, having a long petiole but with its tentacles 
(oi gland-bearing hairs) as yet unfolded, were traced during 
47 h. 15 m. The figure (Fig. 105) shows that it circumnutated 
largely, chiefly in a vertical direction, making two ellipses each 



238 



CIRCUMNUTATION OF LEAVES. 



Chap. IV. 



day. On both days the leaf began to descend after 12 or 
1 o'clock, and continued to do so all night, though to a 

very unequal distance on the 



Fig. 105. 



9'J^a.mJ 




two occasions. We therefore 
thought {hat the movement 
was periodic; but on observ- 
ing three other leaves during 
several successive days and 
nights, we found this to be an 
error; and the case is given 
merely as a caution. On the 
third morning the above leaf 
occupied almost exactly the 
same position as on the first 
morning ; and the tentacles 
by this time had unfolded 
sufficiently to project at right 
angles to the blade or disc. 

The leaves as they grow 
older generally sink more 
and more downwards. The 
movement of an oldish leaf, 
the glands of which were 
still secreting freely, was 
traced for 24 h., during which 
time it continued to sink a 
little in a slightly zigzag line. 
On the following morning, at 
7 a.m., a drop of a solution 
of carbonate of ammonia (2 
gr. to 1 oz. of water) was 
placed on the disc, and this 
blackened the glands and in- 



Drosera rotundifolia : circumnutation 
of young leaf, with filament fixed 
to back of blade, traced from 9.15 
a.m. June 7th to 8.30 A.M. June 
9th. Figure here reduced to one- 
half original scale. 

duced inflection of many of the tentacles. The weight of the 
drop caused the leaf at first to sink a little ; but immediately 
afterwards it began to rise in a somewhat zigzag course, and 
continued to do so till 3 p.m. It then circumnutated about 
the same spot on a very small scale for 21 h. ; and during the 
next 21 h. it sank in a zigzag line to nearly the same level 
which it had held when the ammonia was first administered. 
By this time the tentacles had re-expanded, and the glands had 
recovered their proper colour. We thus learn that an old leal 



Ohap. IV. DICOTYLEDONS. 239 

circumnutates on a small scale, at least whilst absorbing car- 
bonate of ammonia ; for it is probable that this absorption may 
stimulate growth and thus re-excite circumnutation. Whether 
the rising of the glass filament which was attached to the back 
of the leaf, resulted from its margin becoming slightly inflected 
(as generally occurs), or from the rising of the petiole, was not 
ascertained. 

In order to learn whether the tentacles or gland-bearing hairs 
circumnutate, the back of a young leaf, with the innermost 
tentacles as yet incurved, was firmly cemented with shellac 
to a flat stick driven into compact damp argillaceous sand. 
The plant was placed under a microscope with the stage re- 
moved and with an eye- piece micrometer, of which each 
division equalled -^ of an inch. It should be stated that as 
the leaves grow older the tentacles of the exterior rows bend 
outwards and downwards, so as ultimately to become deflected 
considerably beneath the horizon. A tentacle in the second 
row from the margin was selected for observation, and was 
found to be moving outwards at a rate of -^ of an inch in 
20 m., or ^ of inch in 1 h. 40 m. ; but as it likewiso moved 
from side to side to an extent of above gi_ f inch, the move- 
ment was probably one of modified circumnutation. A tentacle 
on an old leaf was next observed in the same manner. In 
15 m. after being placed under the microscope it had moved 
about y^qq of an inch. Daring the next lh h. it was looked at 
repeatedly, and during this whole time it moved only another 
y^qq of an inch ; and this small movement may have been due 
to the settling of the damp sand (on which the plant rested), 
though the sand had been firmly pressed down. We may there- 
fore conclude that the tentacles when old do not circumnutate ; 
yet this tentacle was so sensitive, that in 23 seconds after its 
gland had been merely touched with a bit of raw meat, it began 
to curl inwards. This fact is of some importance, as it appa- 
rently shows that the inflection of the tentacles from the stimulus 
of absorbed animal matter (and no doubt from that of contact 
with any object) is not due to modified circumnutation. 

(15.) Dioncea musciptla (Droseracece). — It should be premised 
that the leaves at an early stage of their development have the 
two lobes pressed closely together. These are at first directed 
back towards the centre of the plant ; but they gradually rise up 
and soon stand at right angles to the petiole, and ultimately in 
nearly a straight line with it. A young leaf, which with t,h« 



240 



CTRCUMNUTAT10N OF LEAVES. 



Chap. IV 



Fig. 106. 



petiole was only 1*2 inch in length, had a filament fixed exter- 
nally along the midrib of the still closed lobes, which projected 
at right angles to the petiole. In the evening this leaf com- 
pleted an ellipse in the course of 2 h. On 
the following day (Sept. 25th) its move- 
ments were traced during 22 h. ; and we 
see in Eig. 106 that it moved in the same 
general direction, due to the straightening 
of the leaf, but in an extremely zigzag line. 
This line represents several drawn-out or 
modified ellipses. There can therefore be 
no doubt that this young leaf circumnu- 
tated. 

A rather old, horizontally extended 
leaf, with a filament attached along the 
under side of the midrib, was next 
observed during 7 h. It hardly moved, 
but when one of its sensitive hairs 
was touched, the blades closed, though 
not very quickly. A new dot was now 
made on the glass, but in the course of 
14 h. 2>J m. there was hardly any change 
in the position of the filament. We may 
therefore infer that an old and only 
moderately sensitive leaf does not circum- 
nutate plainly ; but we shall soon see 
that it by no means follows that such 
a leaf is absolutely motionless. We may 
further infer that the stimulus from a 
touch does not re-excite plain circumnu- 
tation. 

Another full-grown leaf had a filament 
attached externally along one side of the 
midrib and parallel to it, so that the fila- 
ment would move if the lobes closed. It 
should be first stated that, although a touch on one of the sensi- 
tive hairs of a vigorous leaf causes it to close quickly, often 
almost instantly, yet when a bit of damp meat or some solution 
af carbonate of ammonia is placed on the lobes, they close so 
slowly that generally 24 h. is required for the completion of the 
act. The above leaf was first observed for 2 h. 30 m., and did 
not circumnutate, but it ought to have been observed for a 




Doncea muscipuia : cir- 
cumnutation of a 
young and expanding 
leaf, traced on a hori- 
zontal glass in dark- 
ness, from noon Sept. 
24th to 10 A.M. 25th. 
Apex of leaf 13£ 
inches from the glass, 
so tracing consider- 
ably magnified. 



Chap. IV. DICOTYLEDONS 241 

longer period ; although, as we have seen, a young leaf com- 
pleted a fairly large ellipse in 2 h. A drop of an infusion of 
raw meat was then placed on the leaf, and within 2 h. the glass 
filament rose a little ; and this implies that the lobes had begun 
to close, and perhaps the petiole to rise. It continued to rise 
with extreme slowness for the next 8 h. 30 m. The position of 
the pot was then (7.15 p.m., Sept. 24th) slightly changed and 
an additional drop of the infusion given, and a new tracing 
was begun (Fig. 107). By 10.50 p.m. the filament had risen 
only a little more, and it fell during the night. On the follow- 
ing morning the lobes were closing more quickly, and by 5 p.m. 
it was evident to the eye that they had closed considerably ; by 
8.48 p.m. this was still plainer, and by 10.45 p.m. the marginal 
spikes were interlocked. The leaf fell a little during the night, 
and next morning (25th) at 7 am. the lobes were completely 
shut. The course pursued, as may be seen in the figure, was 

Fig. 107. 

Dionasa mu^cipida ; closure of the lobes and circumnutation of a full-grown 
leaf, whilst absorbing an infusion of raw meat, traced in darkness, from 
7.15 p.m. Sept. 24th to 9 a.m. 26th. Apex of leaf 8 J inches from the 
vertical glass. Figure here reduced to two-thirds of original scale. 

strongly zigzag, and this indicates that the closing of the lobes 
was combined with the circumnutation of the whole leaf, 
and there cannot be much doubt, considering how motionless 
the leaf was during 2 h. 30 m. before it received the infusion, 
that the absorption of the animal matter had excited it to 
circumnutate. The leaf was occasionally observed for the next 
four days, but was kept in rather too cool a place ; nevertheless, 
it continued to circumnutate to a small extent, and the lobes 
remained closed. 

It is sometimes staled in botanical works that the lobes close 
or sleep at night ; but this is an error. To test the statement, 
very long glass filaments were fixed inside the two lobes of 
thiee leaves, and the distances between their tips were measured 
in the middle of the day and at night ; but no difference could 
be detected. 

The previous observations relate to the movements of the 
whole leaf, but the lobes move independently of the petiole, and 




242 C1KCUMNUTATI0N OF LEAVES. Chap. IV. 

seem to be continually opening and shutting to a very small 
extent. A nearly full-grown leaf (afterwards proved to be 
highly sensitive to contact) stood almost horizontally, so thai 
by driving a long thin pin through the foliaceous petiole close 
to the blade, it was rendered motionless. The plant, with 
a little triangle of paper attached to one of the marginal spikes, 
was placed under a microscope with an eye-piece micrometer, 
each division of which equalled -^ of an iuch. The apex oi 
the paper-triangle was now seen to be in constant slight move- 
ment ; for in 4 h. it crossed nine divisions, or -^ of an inch, 
and after ten additional hours it moved back and had crossed 
5§o in an opposite direction. The plant was kept in rather 
too cool a place, and on the following day it moved rather less, 
namely, -gfe in 3 h., and t %q in an opposite direction during the 
next 6 h. The two lobes, therefore, seem to be constantly 
closing or opening, though to a very small distance; for we must 
remember that the little triangle of paper affixed to the marginal 
spike increased its length, and thus exaggerated somewhat the 
movement. Similar observations, with the important difference 
that the petiole was left free and the plant kept under a high 
temperature, were made on a leaf, which was healthy, but so old 
that it did not close when its sensitive hairs were repeatedly 
touched, though judging from other cases it would have slowly 
closed if it had been stimulated by animal matter. The apex of 
the triangle was in almost, though not quite, constant movement, 
sometimes in one direction and sometimes in an opposite one ; 
and it thrice crossed five divisions of the micrometer (i.e. y^ of 
an inch) in 30 m. This movement on so small a scale is hardly 
comparable with ordinary circumnutation ; but it may perhaps 
be compared with the zigzag lines and little loops, by which the 
larger ellipses made by other plants are often interrupted. 

In the first chapter of this volume, the remarkable oscillatory 
movements of the circumnutating hypocotyl of the cabbage 
have been described. The leaves of Dionsea present the same 
phenomenon, which is a wonderful one, as viewed under a low 
power (2-inch object-glass), with an eye-piece micrometer oi 
which each division (^^ of an inch) appeared as a rather wide 
space. The young unexpanded leaf, of which the circumnutating 
movements were traced (Fig. 106), had a glass filament fixed 
perpendicularly to it; and the movement of the apex was 
observed in the hot-house (temp. 84° to 86° F.), with light 
admitted only from above, and with any lateral currents of air 



Chat. IV. DICOTYLEDONS. 243 

excluded. The apex sometimes crossed one or two divisions of 
the micrometer at an imperceptibly slow rate, but generally it 
moved onwards by rapid starts or jerks of y^^ or yoVo* an d in 
one instance of toVo °f an inch. After each jerk forwards, the 
apex drew itself backwards with comparative slowness for part 
of the distance which had just been gained ; and then after a 
very short time made another jerk forwards. Four conspi- 
cuous jerks forwards, with slower retreats, were seen on one 
occasion to occur in exactly one minute, besides some minor 
oscillations. As far as we could judge, the advancing and 
retreating lines did not coincide, and if so, extremely minute 
ellipses were each time described. Sometimes the apex remained 
quite motionless for a short period. Its general course during 
the several hours of observation was in two opposite directions, 
so that the leaf was probably circumnutating. 

An older leaf with the lobes fully expanded, and which was 
afterwards proved to be highly sensitive to contact, was next 
observed in a similar manner, except that the plant was exposed 
to a lower temperature in a room. The apex oscillated forwards 
and backwards in the same manner as before ; but the jerks for- 
ward were less in extent, viz. about y^^ inch ; and there were 
longer motionless periods. As it appeared possible that the 
movements might be due to currents of air, a wax taper was 
held close to the leaf during one of the motionless periods, but 
no oscillations were thus caused. After 10 m., however, vigorous 
oscillations commenced, perhaps owing to the plant having been 
warmed and thus stimulated. The candle was then removed and 
before long the oscillations ceased ; nevertheless, when looked at 
again after an interval of 1 h. 30 m., it was again oscillating. 
The plant was taken back into the hot-house, and on the 
following morning was seen to be oscillating, though not very 
vigorously. Another old but healthy leaf, which was rot in the 
least sensitive to a touch, was likewise observed during two 
days in the hot-house, and the attached filament made many 
little jerks forwards of about To ^q or only yJ^ of an inch. 

Finally, to ascertain whether the lobes independently of the 
petiole oscillated, the petiole of an old leaf was cemented close 
to the blade with shellac to the top of a little stick driven into 
the soil. But before this was done the leaf was observed, and 
found to be vigorously oscillating or jerking ; and after it had 
been cemented to the stick, the oscillations of about 10 2 00 of 
an inch stil] continued. On the following day a little infusion 



244 



CIKCUMNUTATION OF LEAVES. Chap. IT 



of raw meat was placed on the leaf, which caused the lobes tc 
close together very slowly in the course of two days ; and the 
oscillations continued during this whole time and for the next 
two days. After nine additional days the leaf began to open 
and the margins were a little everted, and now the apex of the 
glass filament remained for long periods motionless, and then 
moved backwards and forwards for a distance of about y^o of 
an inch, slowly, without any jerks. Nevertheless, after warming 
the leaf with a taper held close to it, the jerking movement 
recommenced. 

This same leaf had been observed 2| months previously, and 
was then found to be oscillating or jerking. We may therefore 
infer that this kind of movement goes on night and day for a 
very long period ; and it is common to young unexpanded leaves 
and to leaves so old as to have lost their sensitiveness to a 
touch, but which were still capable of absorbing nitrogenous 
matter. The phenomenon when well displayed, as in the young 
leaf just described, is a very interesting one. It often brought 
before our minds the idea of effort, or of a small animal 
struggling to escape from some constraint. 

(16.) Eucalyptus resinifera (Myrtacese, Fam. 94). — A young leaf, 
two inches in length together with 
the petiole, produced by a lateral 
shoot from a cut-down tree, was 
observed in the usual manner. 
The blade had not as yet as- 
sumed its vertical position. On 
June 7th only a few observations 
were made, and the tracing merely 
showed that the leaf had moved 
three times upwards and three 
downwards. On the following 
day it was observed more fre- 
quently; and two tracings were 
made (see A and B, Fig. 108), as 
a single one would have been too 
complicated. The apex changed 
its course 13 times in the course 
of 16 h., chiefly up and down, but 
The actual amount of movement 



Fig. 108. 




Eucalyptus resinifera : circumnu- 
tation of a leaf, traced, A, from 
6.40 A.M. to 1 P.M. June 8th ; 
B, from 1 p.m. 8th to 8.30 A.M. 
9th. Apex of leaf 14| inches 
from the horizontal glass, so 
figures considerably magnified. 



with some lateral movement. 
in any one direction was small. 
(17.) Dallia (garden var.) (Composite, Fam. 122). 



-A fine young 



'flAP. IV. 



DICOTYLEDONS. 



245 



leaf 5| inches in length, produced by a young plant 2 feet high, 
growing vigorously in a large pot, was directed at an angle ol 
about 45° beneath the horizon. On June 18th the leaf descended 
from 10 a.m. till 11 35 a.m. (see Fig. 109) ; it then ascended 
greatly till 6 p.m., this ascent being probably due to the light 



Fig. 109. 



J*f35'pmJ8* 
'.Sffa/nm 




Dahlia : circumnutation of leaf, traced from 10 A.M. June 18th to 8.10 a.m. 
20th, but with a break of 1 h. 40 m. on the morning of the 19th, as, 
owing to the glass filament pointing too much to one side, the pot had 
to be slightly moved ; therefore the relative position of the two tracings 
is somewhat arbitrary. The figure here given is reduced to one-fifth of 
the original scale. Apex of leaf 9 inches from the glass in the line 
of its inclination, and 4| in a horizontal line. 

coming only from above. It zigzagged between 6 r.M. and 
10.35 p.m., and ascended a little during the night. It should be 
remarked that the vertical distances in the lower part of the 
diagram are much exaggerated, as the leaf was at first deflected 
beneath the horizon, and after it had sunk downwards, the 
filament pointed in a very oblique line towards the glass. Next 



246 CIRCUMNUTATION OF LEAVES. Chap. IV. 

day the leaf descended from 8.20 a.m. till 7.15 p.m., then zigzagged 
and ascended greatly during the night. On the morning of tbe 
20th the leaf was probably beginning to descend, though the 
short line in the diagram is horizontal. The actual distances 
travelled by the apex of the leaf were considerable, but could 
not be calculated with safety. From the course pursued on the 
second day, when the plant had accommodated itself to the light 
from above, there cannot be much doubt that the leaves undergo 
a daily periodic movement, sinking during the day and rising 
at night. 

(18.) Mutisia clematis (Composite). — The leaves terminate in 
tendrils and circumnutate like those of other tendril-bearers ; 
but this plant is here mentioned, on account of an erroneous 
statement * whioh has been published, namely, that the leaves 
sink at night and rise during the day. The leaves which 
behaved in this manner had been kept for some days in a 
northern room and had not been sufficiently illuminated. A 
plant therefore was left undisturbed in the hot-house, and three 
leaves had their angles measured at noon and at 10 p.m. All 
three were inclined a little beneath the horizon at noon, but one 
stood at night 2°, the second 21°, and the third 10° higher than 
in the middle of the day; so that instead of sinking they rise 
a little at night. 

(19.) Cyclamen Persicum (Primulacese, Fam. 135). — A young 
leaf, 1*8 of an inch in length, petiole included, produced by an 
old root-stock, was observed during three days in the usual 
manner (Fig. 110). On the first day the leaf fell more than after- 
wards, apparently from adjusting itself to the light from above. 
On all three days it fell from the early morning to about 7 p.m., 
and from that hour rose during the night, the course being 
slightly zigzag. The movement therefore is strictly periodic. 
It should be noted that the leaf would have sunk each evening 
a little lower down than it did, had not the glass filament rested 
between 5 and 6 p.m. on the rim of the pot. The amount of 
movement was considerable ; for if we assume that the whole 
leaf to the base of the petiole became bent, the tracing would 
be magnified rather less than five times, and this would give 
to the apex a rise and fall of half an inch, with some lateral 
movement. This amount, however, would not attract attention 
without the aid of a tracing or measurement of some kind. 



The Movements and Habits of Climbing Plants," 1875, p. 118. 



Chap. IV. 



DICOTYLEDONS. 



247 



(20.) Allamanda ScLottii (Apocynese, Fam. 144). — The young 
leaves of tliis shrub are elongated, with the blade bowed so much 



Fig. 110 




7J3.1 



Cyclamen Persicum : circumnutation of leaf, traced from 6.45 A.M June 2ad 
to 6.40 A.M. 5th. Apes of leaf 7 inches from the vertical glass. 

downwards as almost to form a semicircle. The chord — that 
is, a line drawn from the apex of the blade to the base of th6 
petiole — of a young leaf. 4| inches in length, stood at 2.50 p.m od 
17 



248 



CIRCUMNUTATION OF LEAVES. 



Chap. I\> 



Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 p.m. 

the blade had straightened itself 
Fl S- 111 so much, which implies the 

t raising of the apex, that the 

V chord now stood at 37° above the 

J^ horizon, and had therefore risen 

50°. On the next day similar 
angular measurements of the 
same leaf were made; and at 
noon the chord stood 36° be- 
neath the horizon, and 9.30 p.m. 
3|° above it, so had risen 39£°. 
The chief cause of the rising 
movement lies in the straighten- 
ing of the blade, but the short 
petiole rises between 4° and 5°. 
On the third night the chord 
stood at 35° above the horizon, 
and if the leaf occupied the 
same position at noon, as on 
the previous day, it had risen 
71°. With older leaves no such 
change of curvature could be 
detected. The plant was then 
brought into the house and 
kept in a north-east room, but 
at night there was no change 
in the curvature of the young 
leaves; so that previous expo- 
sure to a strong light is appa- 
rently requisite for the periodi- 
cal change of curvature in the 
blade, and for the slight rising 
of the petiole. 

(21.) Wigandia (Hydroleacese, 
Fam. 149).— Professor Pfeffer 
informs us that the leaves of this 
plant rise in the evening ; but as 
we do not know whether or not 
the rising is great, this species 
ought perhaps to be classed 
amongst sleeping plants 





Petunia violacca : downward move- 
ment and circumnutation of a 
very young leaf, traced from 10 
A.M. June 2nd to 9.20 A.M. June 
6th. N.B. — At 6.40 a.m. on the 
5th it was necessary to move the 
pot a little, and a new tracing 
was begun at the point where 
two dots are not joined in the 
diagram. Apex of leaf 7 inches 
from the veitical glass. Temp, 
generally 17J°C. 



Ohap. IV. 



DICOTYLEDONS. 



249 



Fig. 112. 



(22.) Pttunia violacea (Solanese, Fam. 157).— A very young 
leaf, only I inch in length, highly inclined upwards, was observed 
for four days. During the whole of this time it bent outwards 
and downwards, so as to become more and more nearly hori- 
zontal. The strongly marked zigzag line in the figure on p. 248 
(Fig. Ill), shows that this was effected by modified circum- 
nutation ; and during the latter part of the time there was much 
ordinary circumnutation on a small scale. The movement in 
the diagram is magnified between 10 and 11 times. It exhibits 
a clear trace of periodicity, as the leaf rose a little each evening ; 
but this upward tendency appeared to be almost conquered by 
the leaf striving to become more and 
more horizontal as it grew older. The 
angles which two older leaves formed 
together, were measured in the even- 
ing and about noon on 3 successive 
days, and each night the angle de- 
creased a little, though irregularly. 

(23.) Acanthus mollis (Acanthaceae, 
Fam. 168). — The younger of two 
leaves, 2[ inches in length, petiole 
included, produced by a seedling 
plant, was observed during 47 h. 
Early on each of the three morn- 
ings, the apex of the leaf fell ; and 
it continued to fall till 3 p.m., on 
the two afternoons when observed. 
After 3 p.m. it rose considerably, and 
continued to rise on the second night 
until the early morning. But on 
the first night it fell instead of rising, 
and we have little doubt that this 
was owing to the leaf being very 
young and becoming through epi- 
nastic growth more and more hori- 
zontal; for it may be seen in the 
diagram (Fig. 112), that the leaf stood 
on a higher level on the first than on 
fcne second day. The leaves of an 
allied species (A. spinosus) certainly 

rose every night ; and the rise between noon and 10.15 p.m., 
when measured on one occasion, was 10°. This rise was chiefly 




Acanthus mollis: circumnuta- 
tion of young leaf, traced 
from 9.20 A.M. June 14th 
to 8.30 A.M. 16th. Apex 
of leaf 11 inches from the 
vertical glass, so movement 
considerably magnified. 
Figure here reduced toone- 
haif of original Kcale. 
Temp. 15°-16J°C. 



250 CIRCUMNUTATION OF LEAVES. Chap. IV. 

or exclusively due to the straightening of the blade, and not to 
the movement of the petiole. We may therefore conclude that 
the leaves of Acanthus circumnutate periodically, falling in the 
morning and rising in the afternoon and night. 

(24.) Cannabis sativa (Cannabinea?, Fam. 195). — We have 
here the rare case of leaves moving downwards in the evening, 
but not to a sufficient degree to be called sleep.* In the early 
morning, or in the latter part of the night, they move upwards. 
For instance, all the young leaves near the summits of several 
stems stood almost horizontally at 8 a.m. May 29th, and at 
10.30 p.m. were considerably declined. On a subsequent day two 
leaves stood at 2 p.m. at 21° and 12° beneath the horizon, and at 
10 p.m. at 38° beneath it. Two other leaves on a younger plant 
were horizontal at 2 p.m., and at 10 p.m. had sunk to 36° beneath 
the horizon. With respect to this downward movement of the 
leaves, Kraus believes that it is due to their epinastic growth. 
He adds, that the leaves are relaxed during the day, and tense 
at night, both in sunny and rainy weather. 

(25.) Pinus pinuster (Coniferae, Fam. 223). — The leaves on the 
summits of the terminal shoots stand at first in a bundle almost 
upright, but they soon diverge and ultimately become almost 
horizontal. The movements of a young leaf, nearly one inch in 
length, on the summit of a seedling plant only 3 inches high, 
were traced from the early morning of June 2nd to the evening 
of the 7th. During these five days the leaf diverged, and its apex 
descended at first in an almost straight line ; but during the two 
latter days it zigzagged so much that it was evidently circumnu- 
tating. The same little plant, when grown to a height of 5 irjche^, 
was again observed during four days. A filament was fixed 
transversely to the apex of a leaf, one inch in length, and which 
had already diverged considerably from its originally upright 
position. It continued to diverge (see A, Fig. 113), and to 
descend from 11.45 a.m. July 31st to 6.40 a.m. Aug. 1st. On 
August 1st it circumnutated about the same small spaco, and 
again descended at night. Next morning the pot was moved 
nearly one inch to the right, and a new tracing was begun (B), 
From tbis time, viz., 7 a.m. August 2ud to 8.20 a.m. on the 4th 



* We were led to observe this Flcra, 1879, p. G6. We regret that 

plant by Dr. Carl Kraus' paper, we cannot fully understand parti. 

' Beitr'age zur Kentniss der Bcwe- of ibis paper, 
gungen Wachsender Laubblatter,' 



Chap. IV. 



DICOTYLEDONS. 



251 



the leaf manifestly circumnutatcd. It does not appear from the 
diagram that the leaves move periodically, for the descending 
course during the first two nights, was clearly due to epinastic 



1 



Fig. 113. 



6*4iO'a.mJ*. t 




6"40'cf,m.3l^ 




8'20'a.m.4 



-7°a.m.£ v A 



Pinus pinaster: circumnutation of young leaf, traced from 11.45 a.m. 
July JUst to 8.20 A.M.' Aug. 4th. At 7 A.M. Aug. 2nd the pot was 
moved an inch to one side, so that the tracing consists of two figures. 
Apex of leaf 14| inches from the vertical glass, so movements much 
magnified. 

growth, and at the close of our observations the leaf was not 
nearly so horizontal as it would ultimately become. 

Pinus austriaca. — Two leaves, 3 inches in length, but not 



252 



CIBCUMNUTATIGN OF LEAVES. Chap. IV. 



quite fully grown, produced by a lateral shoot, on a young tree 

3 feet in height, were observed during 29 h. (July 31st), in the 

same manner as the leaves of the previous species. Both these 

leaves certainly circumnutafed, making 

Fi S- 114 - within the above period two, or two and 

V a half, small, irregular ellipses. 

(26.) Cycas pectinata (Cycadese, Fam 
224). — A young leaf, 11$ inches in 
length, of which the leaflets had only 
recently become uncurled, was observed 
during 47 h. 30 m. The main petiole 
was secured to a stick at the base of the 
two terminal leaflets. To one of the 
4 latter, 31 inches in length, a filament 

was fixed ; the leaflet was much bowed 
downward, but as the terminal part was 
upturned, the filament projected almost 
horizontally. The leaflet moved (see 
Fig. 114) largely and periodically, for it 
fell until about 7 p.m. and rose during 
the night, falling again next morning 
after 6.40 a.m. The descending lines 
are in a marked manner zigzag, and so 
probably would have been the ascending 
lines, if they had been traced throughout 
the night. 



i 



Oycas pectinata : circura- 
nutation of one of the 
terminal leaflets, traced 
from 8.30 a.m. June 
22nd to 8 A.M. June 
24th. Apex of leaflet 
7| inches from the ver- 
tical glass, so tracing 
not greatly magnified, 
and here reduced to 
one-third of original 
scale; temp. 19°-21°C. 



clrcumnutation of leaves 
Monocotyledons. 



(27.) Canni Warscewiczii (Cannacese, 
Fam. 2). — The movements of a young 
leaf, 8 inches in length and 3£ in 
breadth, produced by a vigorous young 
plant, were observed during 45 h. 
50 in., as shown in Fig. 115. The pot 
was slided about an inch to the right on the morning of the 
11th, as a single figure would have been too complicated; but 
the two figures are continuous in time. The movement is 
periodical, as the leaf descended from the early morning until 
about 5 p M., and ascended during the rest of the evening and 



Chap. IV. 



MONOCOTYLEDONS. 



253 



part of the night. On the evening of the 11th it circuinnutated 
on a small scale for some time about the same spot. 

Fig. 115. 




A. B. 

Canna Warscewiczii • circumnutation of leaf, traced (A) from 11.30 A.M 
June 10th to 6.40 A.M. 11th ; and (B) from 6.40 A.M. 11th to 8.40 A.M. 
12th. Apex of leaf 9 inches from the vertical glass. 

(28.) Iris pseudo-acorus (Iridese, Fam. 10). — The movements 
of a young leaf, rising 13 inches above the water in which the 
plant grew, were traced as shown in the 
figure (Fig. 116), during 27 h. 30 m. 
It manifestly circumnutated, though 
only to a small extent. On the second 
morning, between 6.40 a.m. and 2 p.m. 
(at which latter hour the figure here 
given ends), the apex changed its course 
five times. During the next 8 h. 40 m. it 
zigzagged much, and descended as far 
as the lowest dot in the figure, making 
in its course two very small ellipses ; 
but if these lines had been added to 
the diagram it would have been too 
complex. 

(29.) Crinum Capense (Amaryllideas, 
Fam. 11). — The leaves of this plant 
are remarkable for their great length 
and narrowness: one was measured 
and found to be 53 inches long and 

only T4 broad at the base. Whilst quite young they stand up 
almost vertically to the height of about a foot; afterwards 



Fig. 116. 

7 

Iris pseudo-acorus ; circum- 
nutation of leaf, traced 
from 10.30 A.M. May 28th 
to 2 P.M. 29th. Tracing 
continued to 11 P.M., but 
not here copied. Apex 
of leaf 12 inches beneath 
the horizontal glass, so 
figure considerably mag- 
nified. Temp. 15°-16°C. 



254 CIKCUMNUTATION OF LEAVES. Chap. IV. 

their tips begin to bend over, and subsequenily bang vertically 
down, and thus continue to grow. A rather young leaf was 
selected, of which the dependent tapering point was as yet only 
5 1 inches in length, the upright basal part being 20 inches high, 
though this part would ultimately become shorter by being 
more bent over. A large bell-glass was placed over the plant, 
with a black dot on one side ; and by bringing the dependent 
apex of the leaf into a line with this dot, the accompanying 
figure (Fig. 117) was traced on the other side of the bell, during 
2£ days. During the first day (22nd) the tip travelled laterally 
far to the left, perhaps in consequence of the plant having been 

Fig. 117. 




J2°p.m.23 T : d 

Crinum capense : circumnutation of dependent tip of young leaf, traced „.. 
a bell-glass, from 10.30 p.m. May 22nd to 10.15 A.M. 25th. Figure not 
greatly magnified. 

disturbed ; and the last dot made at 10.30 p.m. on this day is 
alone here given. As we see in the figure, there can be no 
doubt that the apex of this leaf circumnutated. 

A glass filament with little triangles of paper was at the 
same time fixed obliquely across the tip of a still younger leaf, 
which stood vertically up and was as yet straight. Its move- 
ments were traced from 3 p.m. May 22nd to 10.15 a.m. 25th. 
The leaf was growing rapidly, so that the apex ascended greatly 
during this period ; as it zigzagged much it was clearly circum- 
nutating, and it apparently tended to form one ellipse each 
day. The lines traced during the night were much more vertical 
than those traced during the day ; and this indicates that the 
tracing would have exhibited a nocturnal rise and a diurnal 
fall, if the leaf had not grown so quickly. The movement of 
this same leaf after an interval of six days (May 31st), by which 
time the tip had curved outwards into a horizontal position 



Chap. IV. MONOCOTYLEDONS. 255 

and had thus made the first step towards becoming dependent, 
was traced orthogonically by the aid of a cube of wood (in the 
manner before explained) ; and it was thus ascertained that the 
actual distance travelled by the apex, and due to cir cum nutation, 
was 3| inches in the course of 20£ h. During the next 24 h. it 
travelled 2s inches. The circumnutating movement, therefore, 
of this young leaf was strongly marked. 

(30.) Pancratium littorale (Amaryllidese). — The movements, 
much magnified, of a leaf, 9 inches in length and inclined at 
about 45° above the horizon, were traced during two days. On 
the first day it changed its course completely, upwards and 
downwards and laterally, 9 times in 12 h. ; and the figure traced 
apparently represented five ellipsea. On the second day it was 
observed seldomer, and was therefore not seen to change its 
course so often, viz., only 6 times, but in the same complex 
manner as before. The movements were small in extent, but 
there could be no doubt about the circumnutation of the lea£ 

(31.) Imatopliyllum. vel Olivia (sp. ?) (Amaryllidese). — A long 
glass filament was fixed to a leaf, and the angle formed by it 
with the horizon was measured occasionally during three suc- 
cessive days. It fell each morning until between 3 and 4 p.m., 
and rose at night. The smallest angle at any time above the 
horizon was 48°, and the largest 50°; so that it rose only 2° 
at night ; but as this was observed each day, and as similar 
observations were nightly made on another leaf on a distinct 
plant, there can be no doubt that the leaves move periodically, 
though to a very small extent. The position of the apex when 
it stood highest was *8 of an inch above its lowest point. 

(32.) Pistia stratiotes (Aroideae, Fam. 30). — Hofmeister 
remarks that the leaves of this floating water-plant are more 
highly inclined at night than by day.* We therefore fastened 
a fine glass filament to the midrib of a moderately young 
leaf, and on Sept. 19th measured the angle which it formed 
with the horizon 14 times between 9 a.m. and 11.50 p.m. The 
temperature of the hot-house varied during the two days of 
observation between 181° and 232° C. At 9 a.m. the filament 
stood at 32° above the horizon ; at 3.34 p.m. at 10° and at 
11.50 p.m. at 55° ; these two latter angles being the highest and 
the lowest observed during the day, showing a difference of 45°. 
The rising did not become strongly marked until between 



Die Lehre von der Pflanzenzelle,' 18G7, p. 327. 



256 CIRCUMNUTATION OF LEAVES. Chap. IV 

5 and 6 p.m. On the next day the leaf stood at only 10° above 
the horizon at 8.25 a.m., and it remained at about 15° till past 
3 p.m.; at 5.40 p.m. it was 23°, and at 9.30 p.m. 58°; so that 
the rise was more sudden this evening than on the previous 
one, and the difference in the angle amounted to 48°. The 
movement is obviously periodical, and as the leaf stood on the 
first night at 55°, and on the second night at 58° above the 
horizon, it appeared very steeply inclined. This case, as we 
shall see in a future chapter, ought perhaps to have been 
included under the head of sleeping plants. 
(33.) Pontederia (sp. ?) (from the highlands of St. Catharina, 

Fig. 118. 




Pontederia (sp. ?) : circumnutat:on of leaf, traced from 4.50 P.M. July 2nd 
to 10.15 A.M. 4th. Apex of leaf 16J inches from the vertical glass, so 
tracing greatly magnified. Temp, about 17° C, and therefore rather 
too low. 

Brazil) (Pontederiaceae, Fam. 46). — A filament was fixed across 
the apex of a moderately young leaf, 71 inches in height, and 
its movements were traced during 42 k h. (see Fig. 118). On 
the first evening, when the tracing was begun, and during the 
night, the leaf descended considerably. On the next morning 
it ascended in a strongly marked zigzag line, and descended 
again in the evening and during the night. The movement, 
therefore, seems to be periodic, but some doubt is thrown on 
this conclusion, because another leaf, 8 inches in height, 
appearing older and standing more highly inclined, behaved 
differently. During the first 12 h. it circumnutated over a 



Chap. IV. CIBCUMNUTATION OF CEYPTOGAMS. 



257 



small space, but during the night and the whole following day 
it ascended in the same general direction; the ascent being 
effected by repeated up and down well-pronounced oscillations. 

Cryptogams. 

(34.) Nephrodium molle (Filices, Fam. 1). — A filament was 
fixed near the apex of a young frond of this Fern, 17 inches 
in height, which was not as yet fully uncurled ; and its move- 
ments were traced during 24 h. We see in Fig. 119 that it 



Fig. 119. 




Nephrodium molle: circumnutation of rachis, traoed from 9.15 A.M. May 
28th to 9 A.M. 29th. Figure here given two-thirds of original scale. 

plainly circumnutated. The movement was not greatly magnified 
as the frond was placed near to the vertical glass, and would 
probably have been greater and more rapid had the day been 
warmer. For the plant was brought out of a warm greenhouse 
and observed under a skylight, where the temperature was 
between 15° and 16° C. We have seen in Chap. I. that a frond of 
this Fern, as yet only slightly lobed and with a rachis only "23 
inch in height, plainly circumnutated.* 



* Mr. Loomi'S and Prof. Asa 
Gray have described (' Botanical 
Gazette,' 1880, pp. 27, 43), an 
extremely curious case of move- 
ment in the fronds, but only in 
the fruiting fronds, of Asplenium 
trtchomanes. They move almost 
as rapidly as the little leaflets 



of Desmodium gyran?, alternately 
backwards and forwards through 
from 20 1 o 40 degrees, in a plane at 
right angles to that of the frond. 
The apex of the frond describes *' a 
long and very narrow ellipse,'' so 
that it ciroumnutates. But the 
movement differs from ordinary 



258 CIKCUMNUTATION OF CRYPTOGAMS. Chap. 17 



Fig. 120. 



In the chapter on the Sleep of Plants the conspicuous circum- 
nutation of Marsilea quadiijoliata (Marsileacese, Fam. 4) will be 
described. 

It has also been shown in Chap. I. that a very young Sela- 
ginella (Lycopodiaceae, Fam. 6), only "4 inch in height, plainly 
circumnutated; we may therefore conclude that older plants, 
whilst growing, would do the same. 

(35.) Lunularia vulgaris (Hepaticse, Fam. 11, Muscales).— 
The earth in an old flower-pot was 
coated with this plant, bearing 
gemmae. A highly inclined frond, 
which projected "3 inch above the 
soil and was - 4 inch in breadth, was 
selected for observation. A glass 
hair of extreme tenuity, 75 inch 
in length, with its end whitened, 
was cemented with shellac to the 
frond at right angles to its breadth ; 
and a white stick with a minute 
black spot was driven into the soil 
close behind the end of the hair. 
The white end could be accurately 
brought into a line with the black 
spot, and dots could thus be suc- 
cessively made on the vertical 
glass-plate in front. Any move- 
ment of the frond would of course 
be exhibited and increased by the 
long glass hair ; and the black spot 
was placed so close behind the end 
of the hair, relative'y to the dis- 
tance of the g^ass-plate in front, 
that the movement of the end was 
magnified about 40 times: Never- 
theless, we are convinced that onr 
tracing gives a fairly faithful re- 
presentation of the movements of 
the frond. In the intervals between each observation, the plant 
was covered by a small bell-glass. The frond, as already stated, 




Lunularia vulgaris: circumnuta- 
tion of a frond, traced from 
9 A.M. Oct 25th to 8 A.M. 27th. 



rircnmnntation as it occurs only 
when the plant is exposed to the 
light; even artificial light "is 



sufficient to excite motion for a 
few minutes." 



Chap. IV. CIRCUMNUTATION OF LEAVES. 259 

tvas highly inclined, and the pot stood in front of a north-east 
window. During the five first days the frond moved downwards 
or became less inclined; and the long line which was traced 
was strongly zigzag, with loops occasionally formed or nearly 
formed ; and this indicated circumnutation. Whether the sink- 
ing was due to epinastic growth, or apheliotropism, we do not 
know. As the sinking was slight on the fifth day, a new tracing 
was begun on the sixth day (Oct. 25th), and was continued 
for 47 b. ; it is here given (Fig. 120). Another tracing was made 
on the next day (27th) and the frond was found to be still cir- 
cumnutating, for during 14 h. 30 m. it changed its course com- 
pletely (besides minor changes) 10 times. It was casually 
observed for two more days, and was seen to be continually 
moving. 

The lowest members of the vegetable series, the Thallogens, 
apparently circumnutate. If an Oscillaria be watched under 
the microscope, it may be seen to describe circles about every 
40 seconds. After it has bent to one side, the tip first begins 
to bend back to the opposite side and then the whole filament 
curves over in the same direction. Hofmeister* has given a 
minu te account of the curious, but less regular though constant, 
movements of Spirogyra: during 2j h. the filament moved 4 
times to the left and 3 times to the right, and he refers to a 
movement at right angles to the above. The tip moved at the 
rate of about 0T mm. in five minutes. He compares the move- 
ment with the nutation of the higher plants.f We shall hereafter 
see that heliotropic movements result from modified circum- 
nutation, and as unicellular Moulds bend to the light we may 
infer that they also circumnutate. 

Concluding Kemarks on the Circumnutation 
of Leaves. 

The circumnutating movements of young leaves in 
33 genera, belonging to 25 families, widely distributed 



* ' Ueber die Bewegungen der 1880, vol. hi. p. 320) that the 

Faden der Spirogyra princeps: movements of Spirulina, a mem- 

Jahreskefte des Veieins fur vater- ber of the Oscillutorie*, are closely 

landische Naturkunde iu "Wiirt- analogous "to the well-kuown 

tember°:,' 1874, p. 211. rotation of growiug shoots aud 

t Zukal also remarks (as.quoted tendrils." 
in 'Journal R. Mkroscop. Soe.,' 



260 CIECUMNUTATION OF LEAVES. Chap. IV. 

amongst ordinary and gyninospermous Dicotyledons 
and amongst Monocotyledons, together with several 
Cryptogams, have now been described. It would, 
therefore, not be rash to assume that the growing 
leaves of all plants circumnutate, as we nave seen 
reason to conclude is the case with cotyledons. The 
seat of movement generally lies in the petiole, but 
sometimes both in the petiole and blade, or in the 
blade alone. The extent of the movement differed much 
in different plants ; but the distance passed over was 
never great, except with Pistia, which ought perhaps 
to have been included amongst sleeping plants. The 
angular movement of the leaves was only occasionally 
measured ; it commonly varied from only 2° (and pro- 
bably even less in some instances) to about 10°; but 
it amounted to 23° in the common bean. The move- 
ment is chiefly in a vertical plane, but as the ascending 
and descending lines never coincided, there was always 
some lateral movement, and thus irregular ellipses 
were formed. The movement, therefore, deserves tc 
be called one of circumnutation ; for all circumnuta- 
ting organs tend to describe ellipses, — that is, growth 
on one side is succeeded by growth on nearly but not 
quite the opposite side. The ellipses, or the zigzag 
lines representing drawn-out ellipses, are generally 
very narrow ; yet with the Camellia, their minor axes 
were half as long, and with the Eucalyptus more than 
half as long as their major axes. In the case of Cissus, 
parts of the figure more nearly represented circles than 
ellipses. The amount of lateral movement is therefore 
sometimes considerable. Moreover, the longer axes 
of the successively formed ellipses (as with the Bean, 
Cissus, and Sea-kale), and in several instances the 
zigzag lines representing ellipses, were extended in 
very different directions during the same day or on 



Uhap. IV. CIECUMNUTATION OF LEAVES. 2G1 

the next day. The course followed was curvilinear or 
straight, or slightly or strongly zigzag, and little loops 
or triangles were often formed. A single large irregular 
ellipse may be described on one day, and two smaller 
ones by the same plant on the next day. With Drosera 
two, and with Lupinus, Eucalyptus and Pancratium, 
several were formed each day. 

The oscillatory and jerking movements of the leaves 
of Dionaea, which resemble those of the hypocotyl of 
the cabbage, are highly remarkable, as seen under the 
microscope. They continue night and day for some 
months, and are displayed by young unexpanded leaves, 
and by old ones which have lost their sensibility to a 
touch, but which, after absorbing animal matter, close 
their lobes. We shall hereafter meet with the same 
kind of movement in the joints of certain Gramineae, 
and it is probably common to many plants while cir- 
cumnutating. It is, therefore, a strange fact that no 
such movement could be detected in the tentacles of 
Drosera rotundifolia, though a member of the same 
family with Dionsea ; yet the tentacle which was ob- 
served was so sensitive, that it began to curl inwards 
in 23 seconds after being touched by a bit of raw meat. 

One of the most interesting facts with respect to 
the circumnutation of leaves is the periodicity of their 
movements ; for they often, or even generally, rise a 
little in the evening and early part of the night, and 
sink again on the following morning. Exactly the 
same phenomenon was observed in the case of coty^ 
iedons. The leaves in 16 genera out of the 33 which 
were observed behaved in this manner, as did probably 
2 others. Nor must it be supposed that in the remain- 
ing 15 genera there was no periodicity in their move- 
ments ; for 6 of them were observed during too short 
a period for any judgment to be formed on this head 



262 CIECUMNUTATTON OF LEAVES. Chap. IV 

and 3 were so young that their epinastic growth 
which serves to bring them down into a horizontal 
position, overpowered every other kind of movement 
In only one genus, Cannabis, did the leaves sink in 
the evening, and Kraus attributes this movement to 
the prepotency of their epinastic growth. That the 
periodicity is determined by the daily alternations 
of light and darkness there can hardly be a doubt, as 
will hereafter be shown. Insectivorous plants are 
very little affected, as far as their movements are con- 
cerned, by light ; and hence probably it is that their 
leaves, at least in the cases of Sarracenia, Drosera, and 
Diona?a, do not move periodically. The upward move- 
ment in the evening is at first slow, and with different 
plants begins at very different hours ; — with Glaucium 
as early as 11 a.m., commonly between 3 and 5 p.m., 
but sometimes as late as 7 p.m. It should be observed 
that none of the leaves described in this chapter 
(except, as we believe, those of Lupinus speciosus) 
possess a pulvinus; for the periodical movements of 
leaves thus provided have generally been amplified 
into so-called sleep-movements, with which we are not 
here concerned. The fact of leaves and cotyledons 
frequently, or even generally, rising a little in the 
evening and sinking in the morning, is of interest as 
giving the foundation from which the specialised sleep- 
movements of many leaves and cotyledons, not pro- 
vided with a pulvinus, have been developed. The 
above periodicity should be kept in mind, by any one 
considering the problem of the horizontal position of 
leaves and cotyledons during the day, whilst illumi- 
nated from above. 



Chap. V MODIFIED OIKCUMNUTATION. 2fri 



CHAPTER V. 

Modified Circumnutation: Climbing Planjs ; Epinastic and 
Hyponastic Movements. 

Circumnntation modified through innate causes or through the action 
of external conditions — Innate causes— Climbing plants; similarity 
of their movements with those of ordinary plants ; increased ampli- 
tude ; occasional points of difference— Epinastic growth of young 
leaves — Hyponastic growth of the hypoeotyls and epieotyls of seed- 
lings — Hooked tips of climbing and other plants due to modified 
circumuutation — Ampelopsis tricuspidata — Smithia Pfundii — 
Straightening of the tip due to hyponasty — Epinastic growth and 
circunmutation of the flower-peduncles of Trifolium repens and 
Oxalis carnosa. 

The radicles, hypocotyls and epieotyls of seedling 
plants, even before they emerge from the ground, and 
afterwards the cotyledons, are all continually circum- 
nutating. So it is with the stems, stolons, flower- 
peduncles, and leaves of older plants. We may, there- 
fore, infer with a" considerable degree of safety that all 
the growing parts of all plants circumnutate. Although 
this movement, in its ordinary or unmodified state, 
appears in some cases to be of service to plants, 
either directly or indirectly — for instance, the circum- 
nutation of the radicle in penetrating the ground, or 
that of the arched hypocotyl and epicotyl in breaking 
through the surface — yet circumnutation is so general, 
or rather so universal a phenomenon, that we cannot 
suppose it to have been gained for any special pur- 
pose. We must believe that it follows in some un- 
known way from the manner in which vegetable tissues 
grow. 

18 



264 MODIFIED CIRCUMNUTATION. Chap. V 

We shall now consider the many cases in which 
3ircumnutation has been modified for various special 
purposes ; that is, a movement already in progress is 
temporarily increased in some one direction, and tem- 
porarily diminished or quite arrested in other direc- 
tions. These cases may be divided in two sub-classes ; 
in one of which the modification depends on innate or 
constitutional causes, and is independent of external 
conditions, excepting in so far that the proper ones for 
growth must be present. In the second sub-class the 
modification depends to a large extent on external 
agencies, such as the daily alternations of light and 
darkness, or light alone, temperature, or the attraction 
of gravity. The first small sub-class will be considered 
in the present chapter, and the second sub-class in the 
remainder of this volume. 



The Circumnutation of Climbing Plants. 

The simplest case of modified circumnutation is that 
offered by climbing plants, with the exception of 
those which climb by the aid of motionless hooks or 
of rootlets : for the modification consists chiefly in the 
greatly increased amplitude of the movement. This 
would follow either from greatly increased growth over 
a small length, or more probably from moderately in- 
creased growth spread over a considerable length of the 
moving organ, preceded by turgescence, and acting suc- 
cessively on all sides. The circumnutation of climbers 
is more regular than that of ordinary plants ; but in 
almost every other respect there is a close similarity 
between their movements, namely, in their tendency 
to describe ellipses directed successively to all points 
of the compass — in their courses being often inter- 
rupted oy zigzag lines, triangles, loops, or small 






Chap V. CLIMBING PLANTS. 265 

ellipses — in the rate of movement, and in different 
species revolving once or several times within the same 
length of time. In the same internode, the move- 
ments cease first in the lower part and then slowly 
upwards. In both sets of cases the movement mav be 
modified in a closely analogous manner by geotropism 
and by heliotropism ; though few climbing plants are 
jieliotropic. Other points of similarity might be 
pointed out. 

That the movements of climbing plants consist of 
ordinary circumnutation, modified by being increased 
in amplitude, is well exhibited whilst the plants are 
very young ; for at this early age they move like other 
seedlings, but as they grow older their movements 
gradually increase without undergoing any other 
change. That this power is innate, and is not excited 
by any external agencies, beyond those necessary for 
growth and vigour, is obvious. No one doubts that 
this power has been gained for the sake of enabling 
climbing plants to ascend to a height, and thus to 
reach the light. This is effected by two very different 
methods; first, by twining spirally round a support 
but to do so their stems must be long and flexible ; 
and, secondly, in the case of leaf-climbers and tendril- 
bearers, by bringing these organs into contact with a 
support, which is then seized by the aid of their 
sensitiveness. It may be here remarked that these 
latter movements have no relation, as far as we can 
judge, with circumnutation. In other cases the tips 
of tendrils, after having been brought into contact with 
a support, become developed into little discs which 
adhere firmly to it. 

We have said that the circumnutation of climbing 
plants differs from that of ordinary plants chiefly by 
its greater amplitude. But most leaves circumnutate 



266 MODIFIED CIKCUMN CITATION. Chap. V, 

in an almost vertical plane, and therefore describe very 
narrow ellipses, whereas the many kinds of tendrils 
which consist of metamorphosed leaves, make much 
broader ellipses or nearly circular figures ; and thus 
they have a far better chance of catching hold of a 
support on any side. The movements of climbing 
plants have also been modified in some few other 
special ways. Thus the circumnutating stems of Sol- 
nanum dulcamara can twine round a support only 
when this is as thin and flexible as a string or thread. 
The twining stems of several British plants cannot 
twine round a support when it is more than a few 
inches in thickness ; whilst in tropical forests some 
can embrace thick trunks ;* and this great difference 
in power depends on some unknown difference in 
their manner of circumnutation. The most remarkable 
special modification of this movement which we have 
observed is in the tendrils of Echlnoci/stis lobata ; these 
are usually inclined at about 45° above the horizon, 
but they stiffen and straighten themselves so as to 
stand upright in a part of their circular course, namely, 
when they approach and have to pass over the summit 
of the shoot from which they arise. If they had not 
possessed and exercised this curious power, chey would 
infallibly have struck against the summit of the shoot 
and been arrested in their course. As soon as one of 
these Tendrils with its three branches begins to stiffen 
itself and rise up vertically, the revolving motion 
becomes more rapid ; and as soon as it has passed 
over the point of difficulty, its irotion coinciding 
with that from its own weight, causes it to fall into its 
previously inclined position so quickly, that the apex 
can be seen travelling like the hand of a gigantic clock 



'The Movements and Habits of Climbing Plants,' p. 36. 



iTttap. V. EPINASTY AND HYPONASTY. 267 

A large number of ordinary leaves and leaflets and 
a few flower-peduncles are provided with pulvini ; but 
this is not the case with a single tendril at present 
known. The cause of this difference probably lies in 
the fact, that the chief service of a pulvinus is to 
prolong the movement of the part thus provided after 
growth has ceased ; and as tendrils or other climbing- 
organs are of use only whilst the plant is increasing 
in height or growing, a pulvinus which served to 
prolong their movements would be useless. 

It was shown in the last chapter that the stolons or 
runners of certain plants circumnutate largely, and 
that this movement apparently aids them in finding a 
passage between the crowded stems of adjoining plants. 
If it could be proved that their movements had been 
modified and increased for this special purpose, they 
ought to have been included in the present chapter ; 
but as the amplitude of their revolutions is not so 
conspicuously different from that of ordinary plants, 
as in the case of climbers, we have no evidence on 
this head. We encounter the same doubt in the case 
of some plants which bury their pods in the ground. 
This burying process is certainly favoured by the 
circumnutation of the flower-peduncle ; but we do not 
know whether it has been increased for this special 
purpose. 

Epinasty— Hyponasty. 

The term epinasty is used by De Vries * to express 
greater longitudinal growth along the upper than 



* 'Arbciten des Bot. Inst., two terms as first used by Sobim- 

in Wiirzburg,' Heftii. 1872, p. 223. per, and they have been' adopted 

Df- Yries has slightly modified in this sente by Sacks, 
(p. 252) the meaning of the above 



268 MODIFIED CIRCUMNUTATION. Cuaf. V 

along the lower side of a part, which is thus caused tc 
bend downwards ; and hyponasty is used for the reversed 
process, by which the part is made to bend upwards. 
These actions come into play so frequently that the 
use of the above two terms is highly convenient. The 
movements thus induced result from a modified form 
of circumnutation ; for, as we shall immediately see, 
an organ under the influence of epinasty does not 
generally move in a straight line downwards, or under 
that of hyponasty upwards, but oscillates up and down 
with some lateral movement : it moves, however, in a 
preponderant manner in one direction. This shows 
that there is some growth on all sides of the part, but 
more on the upper side in the case of epinasty, and 
more on the lower side in that of hyponasty, than on 
the other sides. At the same time there may be in 
addition, as De Yries insists, increased growth on one 
side due to geotropism, and on another side due to 
heliotropism ; and thus the effects of epinasty or of 
hyponasty may be either increased or lessened. 

He who likes, may speak of ordinary circumnutation 
as being combined with epinasty, hyponasty, the effects 
of gravitation, light, &c. ; but it seems to us, from 
reasons hereafter to be given, to be more correct to 
say that circumnutation is modified by these several 
agencies. We will therefore speak of circumnutation, 
which is always in progress, as modified by epinasty, 
hyponasty, geotropism, or other agencies, whether 
internal or external. 

One of the commonest and simplest cases of epinasty is that 
offered by leaves, which at an early age are crowded together 
round the buds, and diverge as they grow older. Sachs first 
remarked that this was due to increased growth along the upper 
side of the petiole and blade ; and De Vries has now shown in 
more detail that the movement is thus caused, aided slightly by 



Chap. V. E PIN AST Y AND HYPONASTY. 269 

tue weight of the leaf, and resisted as he believes by apogeo- 
tropism, at least after the leaf has somewhat diverged. In our 
observations on the circumnntation of leaves, some were selected 
which were rather too young, so that they continued to diverge 
or sink downwards whilst their movements were being traced. 
This may be seen in the diagrams (Figs. 98 and 112, pp. 232 
and 249) representing the circumnutation of the young leaves ot 
Acanthus mollis and Pelargonium zonale. Similar cases were ob- 
served with Drosera. The movements of a young leaf, only f inch 
in length, of Petunia violacea were traced during four days, and 
offers a better instance (Fig. Ill, p. 248), as it diverged during 
the whole of this time in a curiously zigzag line with some of the 
angles sharply acute, and during the latter days plainly circum- 
nutated. Some young leaves of about the same age on a plant 
of this Petunia, which had been laid horizontally, and on another 
plant which was left upright, both being kept in complete dark- 
ness, diverged in the same manner for 48 h., and apparently 
were not affected by apogeotropism ; though their stems were in 
a state of high tension, for when freed from the sticks to which 
they had been tied, they instantly curled upwards. 

The leaves, whilst very young, on the leading shoots of the 
Carnation (Dianthus caryophyllus) are highly inclined or vertical ; 
and if the plant is growing vigorously they diverge so quickly 
that they become almost horizontal in a day. But they move 
downwards in a rather oblique line and continue for some time 
afterwards to move in the same direction, in connection, we pre- 
sume, with their spiral arrangement on the stem. The course 
pursued by a young leaf whilst thus obliquely descending was 
traced, and the line was distinctly yet not strongly zigzag ; the 
larger angles formed by the successive lines amounting only to 
135°, 154°, and 163°. The subsequent lateral movement (shown 
in Fig. 96, p. 231) was strongly zigzag with occasional circum- 
nutations. The divergence and sinking of the young leaves 
of this plant seem to be very little affected by geotropism or 
heliotropism ; for a plant, the leaves of which were growing 
rather slowly (as ascertained by measurement) was laid hori- 
zontally, and the opposite young leaves diverged from one 
another symmetrically in the usual manner, without any up- 
turning in the direction of gravitation or towards the light. 

The needle-like leaves of Pinus pinaster form a bundle whilst 
young ; afterwards they slowly diverge, so that those on the up- 
right shoots become horizontal. The movements of one such 



270 



MODIFIED CIRCUMNUTATIOX. 



Chap. V 



young leaf was traced during 4 \ days, and the tracing here given 
(Fig. 121) shows that it descended at first in a nearly straight 

line, but afterwards zigzagged, 



Fig. 121. 




Vhvj*, frhister : epinastic downward 
movernent of a young leaf, pro- 
duced by a young plant in a pot, 
traced on a vertical glass under a 
8Ky light, from 6.45 a.m. June 2nd 
to' 10.40 P.M. Gth. 



making one or two little loops. 
The diverging and descend- 
ing movements of a rather 
older leaf were also traced 
(see former Fig. 113, p. 251) : 
it descended during the first 
day and night in a some- 
what zigzag line ; it then cir- 
eumnutated round a small 
space and again descended. 
By this time the leaf had 
nearly assumed its final posi- 
tion, and now plainly circuni- 
nutated. As in the case of the 
Carnation, the leaves, whilst 
very young, do not seem to be 
much affected by geotropism 
or heliotropism, for those on a 
young plant laid horizontally, 
and those on another plant 
left upright, both kept in the 
dark, continued to diverge in 
the usual manner without 
bending to either side. 

With. Cuhcea scandens, the 
young leaves, as they succes- 
sively diverge from the lead- 
ing shoot which, is bent to 
one side, rise up so as to pro- 
ject vertically, and they retain 
this position for some time 
whilst the tendril is revolving. 
The diverging and ascending 
movements of the petiole of 
one such a leaf, were traced on 
a vertical glass under a sky- 
light ; and the course pursued 
was in most parts nearly 
straight, but there were twc 



Chap. V. EPINASTY AND HYPONASTY. 271 

well-marked zigzags (one of theni forming an angle of 112°) , 
and this indicates circumnutation. 

The still closed lobes of a young leaf of Dionrea projected at 
right angles to the petiole, and were in the act of slowly rising. 
A glass filament was attached to the under side of the midrib, 
and its movements were traced on a vertical glass. It circum- 
nutated once in the evening, and on the next day rose, as already 
described (see Fig. 106, p. 240), by a number of acutely zigzag 
lines, closely approaching in character to ellipses. This move- 
ment no doubt was due to epinasty, aided by apogeotropism, 
for the closed lobes of a very young leaf on a plant which had 
been placed horizontally, moved into nearly the same line with 
the petiole, as if the plant had stood upright ; but at the same 
time the lobes curved laterally upwards, and thus occupied an 
unnatural position, obliquely to the plane of the foliaceous petiole. 

As the hypocotyls and epicotyls of some plants protrude from 
the seed-coats in an arched form, it is doubtful whether the 
arching of these parts, which is invariably present when they 
break through the ground, ought always to be attributed to 
epinasty; but when they are at first straight and afterwards 
become arched, as often happens, the arching is certainly due to 
epinasty. As long as the arch is surrounded by compact earth 
it must retain its form; but as soon as it rises above the 
surface, or even before this period if artificially freed from the 
surrounding pressure, it begins to straighten itself, and this no 
doubt is mainly due to hyponasty. The movement of the 
upper and lower half of the arch, and of the crown, was occa- 
sionally traced ; and the course was more or less zigzag, showing 
modified circumnutation. 

With not a few plants, especially climbers, the summit of the 
shoot is hooked, so that the apex points vertically downwards. 
In seven genera of twining plants * the hooking, or as it has been 
called by Sachs, the nutation of the tip, is mainly due to an 
exaggerated form of circumnutation. That is, the growth is so 
great along one side that it bends the shoot completely over to 
the opposite side, thus forming a hook; the longitudinal line or 
zone of growth then travels a little laterally round the shoot, 
and the hook points in a slightly different direction, and so 
onwards until the hook is completely reversed. Ultimately it 



* ' The Movements and Habits of Climbing Plants,' 2nd edit. p. 13. 



'SI 2 MODIFIED CIECUMNUTATION. Chap. V. 

comes back to the point whence it started. This was ascertained 
by painting narrow lines with Indian ink along the convex 
surface of several hooks, and the line was found slowly to be- 
come at first lateral, then to appear along the concave surface, 
and ultimately back again on the convex surface. In the case of 
Lonictra brachypoda the hooked terminal part of the revolving 
shoot straightens itself periodically, but is never reversed ; that 
is, the periodically increased growth of the concave side of the 
hook is sufficient only to straighten it, and not to bend it over 
to the opposite side. The hooking of the tip is of service to 
twining plants by aiding them to catch hold of a support, and 
afterwards by enabling this part to embrace the support much 
more closely than it could otherwise have done at first, thus 
preventing it, as we often observed, from being blown away by a 
strong wind. Whether the advantage thus gained by twining 
plants accounts for their summits being so frequently hooked, 
we do not know, as this structure is not very rare with plants 
which do not climb, and with some climbers (for instance, Vitis, 
Ampelopsis, Cissus, &c.) to whom it does not afford any assist- 
ance in climbing. 

With respect to those cases in which the tip remains always 
bent or hooked towards the same side, as in the genera just 
named, the most obvious explanation is that the bending is due 
to continued growth in excess along the convex side. Wiesner, 
however, maintains * that in all cases the hooking of the tip is 
the result of its plasticity and weight, — a conclusion which from 
what we have already seen with several climbing plants is 
certainly erroneous. Nevertheless, we fully admit that the 
weight of the part, as well as geotropism, &c, sometimes come 
into play. 

Ampelopsis tricu^pidata. — This plant climbs by the aid of 
adhesive tendrils, and the hooked tips of the shoots do not 
appear to be of any service to it. The hooking depends chiefly, 
as far as we could ascertain, on the tip being affected by epinasty 
and geotropism ; the lower and older parts continually straight- 
ening themselves through hyponasty and apogeotropism. We 
believe that the weight of the apex is an unimportant element, 
because on horizontal or inclined shoots the hook is often 
extended horizontally or even faces upwards. Moreover shoots 
frequently form loops instead of hooks; and in this case the 



Sitzb. der k. Akad. dcr Wissensch.,' Yiennn, Jan. 1880, p. 1G. 



Chap. Y. 



EFINASTY AND HYPONASTY. 



273 



extreme- part, instead of hang- 
ing vertically down as would 
follow if weight was the efficient 
cause, extends horizontally or 
even points upwards. A shoot, 
which terminated in a rather 
open hook, was fastened in 
a highly inclined downward 
position, so that the concave 
side faced upwards, and the 
result was that the apex at first 
curved upwards. This ap- 
parently was due to epinasty 
and not to apogeotropism, for 
the apex, soon after passing 
the perpendicular, curved so 
rapidly downwards that we 
could not doubt that the move- 
ment was at least aided by 
geotropism. In the course of 
a few hours the hook was thus 
converted into a loop with the 
apex of the shoot pointing 
straight downwards. The 
longer axis of the loop was at 
first horizontal, but after- 
wards became vertical. During 
this same time the basal part 
of the hook (and subsequently 
of the loop) curved itself slowly 
upwards ; and this must have 
been wholly due to apogeo- 
tropism in opposition to hypo- 
nasty. The loop was then 
fastened upside down, so that 
its basal half would be simul- 
taneously acted on by hypo- 
nasty (if present) and by apo- 
geotropism; and now it curved 
itself so greatly upwards in 
the course of only 4h. that 
there could hardly be a doubt 
that both forces were acting 



Fi S- I 22 - *Wt.^ 



/ 



ld°50'a«ril4 ^f^V/>«<&S^ 




10*.l&JX7)lJ£fr 



t9j>.m.' 



^fSrjS'ctm. 



8Wa.ml3 



4 



Ampelopsis tricuspidata : hvponnstic 
movement of hooked tip of leading 
shoot, traced from 8.10 A.M. July 
13th to 8 A.M. 15th. Apex of shoot 
5g inches from the vertical glass. 
Plant illuminated through a sky- 
light. Temp. 17t c -19°C. Diagram 
reduced to one-third of ori^ina"; scale- 



274 



MODIFIED CIRCUMNUTATION. 



CiiAr. V. 



Kg. 123. JMPIgjJ. 



B'aAn.lO^ 




•Jl°miv. 



fr&aanJS* 



6'd& } cx.m.l3t?L 



C?a.ml2': 



a 



Smithia Pfnndh: hyponastic movement 
of the curved summit of astern, whilst 
straightening itself, traced from 9 
A.M. July 10th to 3 P.M. 13th. Apex 
9£ inches from the vertical glass. 
Diagram reduced to one-fifth of 
original scale. Plant illuminated 
through skylight ; temp. 17£°-19° C. 



together. At the same time 
the loop became open and 
was thus reconverted into a 
hook, and this apparently 
was effected by the geotropic 
movement of the apex in 
opposition to epinasty. In 
the case of Ampelopsis hede- 
racea, weight plays, as far as 
we could judge, a more im- 
portant part in the hooking 
of the tip. 

In order to ascertain 
whether the shoots of A. tri- 
cuspiduta in straightening 
themselves under the com- 
bined action of hyponasty and 
apogeotropism moved in a 
simple straight course, or 
whether they circumnutated, 
glass filaments were fixed to 
the crowns of four hooked 
tips standing in their natural 
position ; and the movements 
of the filaments were traced 
on a vertical glass. All four 
tracings resembled each other 
in a general manner ; but we 
will give only one (see Fig. 
122, p. 273). Tha filament 
rose at first, which shows 
that the hook was straighten- 
ing itself; it then zigzagged, 
moving a little to the left 
between 9.25 a.m. and 9 p.m. 
From this latter hour on the 
13th to 10.50 a.m. on the fol- 
lowing morning (14th) the 
hook continued to straighten 
itself, and then zigzagged a 
short distance to the right. 
But from 1 p.m. to 10.40 p.m. 
on the 14th the movement 



iJiiai'. V. EPINASTY AND HYPONASTY. 275 

was reversed and the shoot became more hooked. During 
the night, after 10.40 p.m. to 8.15 a.m. on the 15th, the hook 
again opened or straightened itself. By this time the glass 
filament had become so highly inclined that its movements could 
no longer be traced with accuracy; and by 1.30 p.m. on this same 
day, the crown of the former arch or hook had become perfectly 
straight and vertical. There can therefore be no doubt that the 
straightening of the hooked shoot of this plant is effected by 
the circumnutation of the arched portion — that is, by growth 
alternating between the upper and lower surface, but prepon- 
derant on the lower surface, with some little lateral movement. 

We were enabled to trace the movement of another straight- 
ening shoot for a longer period (owing to its slower growth and 
to its having been placed further from the vertical glass), namely, 
from the early morning on July 13th to late in the evening of the 
16th. During the whole daytime of the 14th, the hook straight- 
ened itself very little, but zigzagged and plainly circumnutated 
about nearly the same spot. By the 16th it had become nearly 
straight, and the tracing was no longer accurate, yet it was 
manifest that there was still a considerable amount of movement 
both up and down and laterally; for the crown whilst con- 
tinuing to straighten itself occasionally became for a short time 
more curved, causing the filament to descend twice during the 
day. 

ISmithia Pfandii. — The stiff terminal shoots of this Legu- 
minous water-plant from Africa project so as to make a rectangle 
with the stem below ; but this occurs only when the plants are 
growing vigorously, for when kept in a cool place, the summits 
of the stems become straight, as they likewise did at the close 
of the growing season. The direction of the rectangularly bent 
part is independent of the chief source of light. But from 
observing the effects of placing plants in the dark, in which 
case several shoots became in two or three days upright or nearly 
upright, and when brought back into the light again became 
rectangularly curved, we believe that the bending is in part 
due to apheliotropism, apparently somewhat opposed by apogeo- 
tropism. On the other hand, from observing the effects of tying 
a shoot downwards, so that the rectangle faced upwards, we are 
led to believe that the curvature is partly due to epinasty. As 
the rectangularly bent portion of an upright stem grows older, 
the lower part straightens itself; and this is effected through 
hyponasty. He who has read Sachs' recent Essay on the vertical 



276 MODIFIED CIECUMNUTATION. Chap. V 

and inclined positions of the parts of plants* will see how diffi- 
cult a subject this is, and will feel no surprise at our expressing 
ourselves doubtfully in this and other such cases. 

A plant, 20 inches in height, was secured to a stick close 
beneath the curved summit, which formed rather less than a 
rectangle with the stem below. The shoot pointed away from the 
observer ; and a glass filament pointing towards the vertical glass 
on which the tracing was made, was fixed to the convex surface of 
the curved portion. Therefore the descending lines in the figure 
represent the straightening of the curved portion as it grew 
older. The tracing (Fig. 123, p. 274) was begun at 9 a.m. on 
July 10th; the filament at first moved but little in a zigzag line, 
but at 2 p.m. it began rising and continued to do so till 9 p.m. ; 
and this proves that the terminal portion was being more bent 
downwards. After 9 p.m. on the 10th an opposite movement 
commenced, and the curved portion began to straighten itself, 
?nd this continued till 11.10 a.m. on the 12th, but was interrupted 
by some small oscillations and zigzags, showing movement in 
different directions. After 11.10 a.m. on the 12th tins part of 
the stem, still considerably curved, circumnutated in a con- 
spicuous manner until nearly 3 p.m. on the 13th; but during all 
this time a downward movement of the filament prevailed, 
caused by the continued straightening of the stem. By the 
afternoon of the 13th, the summit, which had originally been 
deflected more than a right angle from the perpendicular, had 
grown so nearly straight that the tracing could no longer be 
continued on the vertical glass. There can therefore be no 
doubt that the straightening of the abruptly curved portion of 
the growing stem of this plant, which appears to be wholly due 
to hyponasty, is the result of modified circumnutation. "We 
will only add that a filament was fixed in a different manner 
across the curved summit of another plant, and the same general 
kmd of movement was observed. 

Trifolium reruns. — In many, but not in all the species of Tri- 
folium, as the separate little flowers wither, the sub-peduncles 
bend downwards, so as to depend parallel to the upper part of 
the main peduncle. In Tr. subtcrraneum the main peduncle 
curves downwards for the sake of burying its capsules, and in 
this species the sub-peduncles of the separate flowers bend 



* ' Uebei Orthotrope unci PI a- ten dee Bot. Inst., in Wiirzburg, 1 
gioti-ope PilanzeutUL'ile;' 'Arbei- Heft ii. 1S79, p. 22ti. 



CJtiAP. V. 



EPINASTY AND HYPONASTY. 



2? 



Fig. 124. 



>nci 




8?pt.f!< 



teWama*! 




Trifeliwm repens : circumnu- 
tating and epinastic move- 
ments of the sub-peduncle 
of a single flower, traced 
on a vertical glass under 
a skylight, in A from 11.30 
A.M. Aug. 27th to 7 A.M. 
30th ; in B from 7 a.m. 
Aug. 30th to a little after 
6 p.m. Sept. 8th. 



rn8 l 



278 MODIFIED CIRCUMNUTATION. CHAr. V. 

upwards, so as to occupy the same position relatively to the 
upper part of the main peduncle as in tr. re-pens. This fact 
alone would render it probable that the movements of the sub- 
peduncles in Tr. repens were independent of geotropism. Never 
theless, to make sure, some flower-heads were tied to little sticks 
upside down and others in a horizontal position ; their sub- 
peduncles, however, all quickly curved upwards through the 
action of heliotropism. We therefore protected some flower- 
heads, similarly secured to sticks, from the light, and although 
some of them rotted, many of their sub-peduncles turned very 
slowly from their reversed or from their horizontal positions, 
so as to stand in the normal manner parallel to the upper part 
of the main peduncle. These facts show that the movement is 
independent of geotropism or apheliotropism ; it must there- 
be attributed to epinasty, which however is enecked, at least as 
long as the flowers are young, by heliotropism. Most of the 
above flowers were never fertilised owing to the exclusion of 
bees ; they consequently withered very slowly, and the movements 
of the sub-peduncles were in like manner much retarded. 

To ascertain the nature of the movement of the sub-peduncle, 
whilst bending downwards, a filament was fixed across the 
summit of the calyx of a not fully expanded and almost upright 
flower, nearly in the centre of the head. The main peduncle 
was secured to a stick close beneath the head. In order to see 
the marks on the glass filament, a few flowers had to be cut 
away on the lower side of the head. The flower under obser- 
vation at first diverged a little from its upright position, so as 
to occupy the open space caused by the removal of the adjoining 
flowers. This required two days, after which time a new tracing- 
was begun (Fig. 124). In A we see the complex circumnutating 
course pursued from 11.30 a.m. Aug. 26th to 7 a.m. on the 
30th. The pot was then moved a very little to the right, and 
the tracing (B) was continued without interruption from 7 a.m, 
Aug. 30th to after 6 p.m. Sept. 8th. It should be observed that 
on most of these days, only a single dot was made each morning 
at the same hour. Whenever the flower was observed carefully, 
as on Aug. 30th and Sept. 5th and 6 th, it was found to be cir- 
cumnutating over a small space. At last, on Sept. 7th, it 
began to bend downwards, and continued to do so until after 
6 p.m. on the 8th, and indeed until the morning of the 9th, when 
its movements could no longer be traced on the vertical glass. 
It was carefully observed during the whole of the 8th, and by 



Csap. V. EPINASTY AND HYFONASTY. 279 

10.30 p.m. it had descended to a point lower down by two- thirds 
of the length of the figure as here given ; but from want of space 
the tracing has been copie 1 in B, only to a little after 6 p.m. On 
the morning of the 9th the flower was withered, and the sub- 
peduncle now stood at an angle of 57° beneath the horizon. If 
the flower had been fertilised it would have withered much 
sooner, and have moved much more quickly. We thus see that 
the sub-peduncle oscillated up and down, or circumnutattd, 
during its whole downward epinastic course. 

The sub-peduncles of the fertilised and withered flowers 
of Oxalis carnosa likewise bend downwards through epinasty, 
as will be shown in a future chapter; and theii downward 
course is strongly zigzag, indicating circumnutation. 

The number of instances in which various organs 
move through epinasty or hyponasty, often in com- 
bination with other forces, for the most diversified 
purposes, seems to be inexhaustibly great ; and from 
the several cases which have been here given, we may 
safely infer that such movements are due to modified 
circumnutation. 



19 



280 MODIFIED CIRCUMNUTATION. Chap. VI 



CHAPTER VI. 

Modified Circumntjtation : Sleep or Nyctitropic Movements, 
their Use: Sleep op Cotyledons. 

Preliminary sketch of the sleep or nyctitropic movements of leaves — 
Presence of pulvini — The lessening of radiation the final cause of 
nyctitropic movements — Manner of trying experiments on leaves of 
Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, and on the 
cotyledons of Mimosa — Concluding remarks on radiation from leaves 
— Small differences in the conditions make a great difference in the 
result — Description of the nyctitropic position and movements of 
the cotyledons of various plants — List of species — Concluding 
remarks — Independence of the nyctitropic movements of the leaves 
and cotyledons of the same species — Eeasons for believing that the 
movements have been acquired for a special purpose. 

The so-called sleep of leaves is so conspicuous a 
phenomenon that it was observed as early as the 
time of Pliny ;* and since Linnaeus published his 
famous Essay, i Somnus Plantarum,' it has been the 
subject of several memoirs. Many flowers close at 
night, and these are likewise said to sleep ; but we 
are not here concerned with their movements, for 
although effected by the same mechanism as in the 
case of young leaves, namely, unequal growth on the 
opposite sides (as first proved by Pfeffer), yet they differ 
essentially in being excited chiefly by changes of 
temperature instead of light ; and in being effected, as 
far as we can judge, for a different purpose. Hardly 
any one supposes that there is any real analogy 



* Pfeffer has given a clear and riodischen Bewegungcn der Blat- 
interesting sketch of the history torgarie,' 1875, p. 1G3 
Df this subject in his 'Die Pe- 



Chap. VI. SLEEP MOVEMENTS. 281 

between the sleep of animals and that of plants,* 
whether of leaves or flowers. It seems, therefore, 
advisable to give a distinct name to the so-called 
sleep-movements of plants. These have also generally 
been confounded, under the term " periodic," with the 
slight daily rise and fall of leaves, as described in the 
fourth chapter ; and this makes it all the more desir- 
able to give some distinct name to sleep-movements. 
.Nyctitropism and nyctitropic, i.e. night-turning, may 
be applied both to leaves and flowers, and will be 
occasionally used by us ; but it would be best to con- 
fine the term to leaves. The leaves of some few plants 
move either upwards or downwards when the sun shines 
intensely on them, and this movement has sometimes 
been called diurnal sleep ; but we believe it to be of 
an essentially different nature from the nocturnal 
movement, and it will be briefly considered in a 
future chapter. 

The sleep or nyctitropism of leaves is a large 
subject, and we think that the most convenient plan 
will be first to give a brief account of the position 
which leaves assume at night, and of the advantages 
apparently thus gained. Afterwards the more re- 
markable cases will be described in detail, with 
respect to cotyledons in the present chapter, and to 
leaves in the next chapter. Finally, it will be shown 
that these movements result from circumnutation, 
much modified and regulated by the alternations of 
day and night, or light and darkness ; but that they 
are also to a certain extent inherited. 

Leaves, when they go to sleep, move either upwards 
or downwards, or in the case of the leaflets of coni- 



* Ch. Eoyer must, however, be Nat.' (5th series), Bot. vol, ix 
excepted ; see ' Annales des Sc. 1868, p. 378. 



282 MODIFIED ClllCUMNUTATION. Cuap. VI 

pound leaves, forwards, that is, towards the apex of the 
leaf, or backwards, that is, towards its base ; or, again, 
they may rotate on their own axes without moving 
either upwards or downwards. But in almost every 
case the plane of the blade is so placed as to stand 
nearly or quite vertically at night. Therefore the apex, 
or the base, or either lateral edge, may be directed 
towards the zenith. Moreover, the upper surface of 
each leaf, and more especially of each leaflet, is often 
brought into close contact with that of the opposite 
one ; and this is sometimes effected by singularly 
complicated movements. This fact suggests that the 
upper surface requires more protection than the lower 
one. For instance, the terminal leaflet in Trifolium, 
after turning up at night so as to stand vertically, 
often continues to bend over until the upper surface is 
directed downwards whilst the lower surface is fully 
exposed to the sky ; and an arched roof is thus 
formed over the two lateral leaflets, which have their 
upper surfaces pressed closely together. Here we have 
the unusual case of one of the leaflets not standing 
vertically, or almost vertically, at night. 

Considering that leaves in assuming their nycti- 
tropic positions often move through an angle of 
90°; that the movement is rapid in the evening; 
that in some cases, as Ave shall see in the next 
chapter, it is extraordinarily complicated ; that with 
certain seedlings, old enough to bear true leaves, 
the cotyledons move vertically upwards at night, 
whilst at the same time the leaflets move ver- 
tically downwards ; and that in the same genus 
the leaves or cotyledons of some species move 
upwards, whilst those of other species move down- 
wards ; — from these and other such facts, it is hardly 
possible to doubt that plants must derive some 



Chap. VI SLEEP MOVEMENTS. 283 

great advantage from such remarkable powers oi 
movement. 

The nyctitropic movements of leaves and cotyledons 
are effected in two ways,* firstly, by means of pulvini 
which become, as PfefTer has shown, alternately more 
turgescent on opposite sides ; and secondly, by in- 
creased growth along one side of the petiole or 
midrib, and then on the opposite side, as was first 
proved by Batalin.t But as it has been shown by 
De Yries t that in these latter cases increased growth 
is preceded by the increased turgescence of the cells, 
the difference between the above two means of move- 
ment is much diminished, and consists chiefly in the 
turgescence of the cells of a fully developed pulvinus, 
not being followed by growth. When the move- 
ments of leaves or cotyledons, furnished with a pul- 
vinus and destitute of one, are compared, they are seen 
to be closely similar, and are apparently effected for 
the same purpose. Therefore, with our object in view, 
it does not appear advisable to separate the above two 
sets of cases into two distinct classes. There is, how- 
ever, one important distinction between them, namely, 
that movements effected by growth on the alternate 
sides, are confined to young growing leaves, whilst those 
effected by means of a pulvinus last for a long time. 
We have already seen well-marked instances of this 
latter fact with cotyledons, and so it is with leaves, as 
has been observed by PfefTer and by ourselves. The 
long endurance of the nyctitropic movements when 
effected by the aid of pulvini indicates, in addition tc 
the evidence already advanced, the functional irnnort- 



* This distinction was first Dassen in 1837. 

pointed out (according to Pfeffer, t ' Flora,' 1873, p. 433. 

'Die Periodischen Bewegungen X 'Bet. Zeitung/ 1879, Dec 

der Blattorgane,' 1875, p. 161) by 19 th, p. 830. 



284 MODIFIED CIRCUMN CITATION. Chap. VI 

ance of such movements to the plant. There is another 
difference between the two sets of cases, namely, that 
there is never, or very rarely, any torsion of the 
leaves, excepting when a pulvinus is present ; * but 
this statement applies only to periodic and nyctitropic 
movements, as may be inferred from other cases given 
by Frank.f 

The fact that the leaves of* many plants place 
themselves at night in widely different positions from 
what they hold during the day, but with the one 
point in common, that their upper surfaces avoid 
facing the zenith, often with the additional fact that 
they come into close contact with opposite leaves or 
leaflets, clearly indicates, as it seems to us, that the 
object gained is the protection of the upper sur- 
faces from being chilled at night by radiation. There 
is nothing improbable in the upper surface needing 
protection more than the lower, as the two differ in 
function and structure. All gardeners know that 
plants suffer from radiation. It is this and not 
cold winds which the peasants of Southern Europe 
fear for their olives.J Seedlings are often protected 
from radiation by a very thin covering of straw ; and 
fruit-trees on walls by a few fir-branches, or even by a 
fishing-net, suspended over them. There is a variety 
of the gooseberry,§ the flowers of which from being 
produced before the leaves, are not protected by 
them from radiation, and consequently often fail to 
yield fruit. An excellent observer || has remarked 

* Pfeffer, ' Die Period. Beweg. Dew/ remarks that an exposed 

der Blattore;ane,' 1875, p. 159. thermometer rises as soon as even 

f 'Die Nat. Wagerechte Rich- a fleecy cloud, high in the sky, 

tung von Pflanzentheilen,' 1870, passes over the zenith. 

p. 52. § 'Loudon's Gardener's Mag.,' 

% Martins in 'Bull. Soe. Bot. vol. iv. 1828, p. 112. 

de France,' torn. xix. 1872. [| Mr. Rivers in ■ Gardener's 

Wells, in his famous ' Essay on Chron.,' 1866, p. 732. 



Chap. VI. USE OF SLEEP MOVEMENTS. 28£ 

that one variety of the cherry has the petals of its 
flowers much curled backwards, and after a severe 
frost all the stigmas were killed ; whilst at the same 
time, in another variety with incurved petals, the 
stigmas were not in the least injured. 

This view that the sleep of leaves saves them from 
being chilled at night by radiation, would no doubt 
have occurred to Linnreus, had the principle of radia- 
tion been then discovered ; for he suggests in many 
parts of his * Somnus Plantarum ' that the position of 
the leaves at night protects the young stems and 
buds, and often the young inflorescence, against cold 
winds. We are far from doubting that an additional 
advantage may be thus gained ; and we have observed 
with several plants, for instance, Desmodium gyrans, 
that whilst the blade of the leaf sinks vertically down at 
night, the petiole rises, so that the blade has to move 
through a greater angle in order to assume its vertical 
position than would otherwise have been necessary ; but 
with the result that all the leaves on the same plant 
are crowded together as if for mutual protection. 

We doubted at first whether radiation would affect 
in any important manner objects so thin as are many 
cotyledons and leaves, and more especially affect dif- 
ferently their upper and lower surfaces ; for although 
the temperature of their upper surfaces would un- 
doubtedly fall when freely exposed to a clear sky, yet 
we thought that they would so quickly acquire by 
conduction the temperature of the surrounding air, 
that it could hardly make any sensible difference 
to them, whether they stood horizontally and radiated 
into the open sky, or vertically and radiated chiefly 
in a lateral direction towards neighbouring plants and 
other objects. We endeavoured, therefore, to ascer- 
tain something on this head by preventing the leaves 



286 MODIFIED CIECUMNUTATION. Chap. VI 

of several plants from going to sleep, and by exposing 
to a clear sky when the temperature was beneath 
the freezing-point, these, as well as the other leaves 
on the same plants which had already assumed their 
nocturnal vertical position. Our experiments show 
that leaves thus compelled to remain horizontal at 
night, suffered much more injury from frost than 
those which were allowed to assume their normal 
vertical position. It may, however, be said that 
conclusions drawn from such observations are not 
applicable to sleeping plants, the inhabitants of 
countries where frosts do not occur. But in every 
country, and at all seasons, leaves must be exposed to 
nocturnal chills through radiation, which might be in 
some degree injurious to them, and which they would 
escape by assuming a vertical position. 

In our experiments, leaves were prevented from 
assuming their nyctitropic position, generally by 
being fastened with the finest entomological pins 
(which did not sensibly injure them) to thin sheets 
of cork supported on sticks. But in some instances 
they were fastened down by narrow strips of card, 
and in others by their petioles being passed through 
slits in the cork. The leaves were at first fastened 
close to the cork, for as this is a bad conductor, and as 
the leaves were not exposed for long periods, we thought 
that the cork, w r hich had been kept in the house, would 
very slightly warm them ; so that if they were injured 
by the frost in a greater degree than the free vertical 
leaves, the evidence would be so much the stronger 
that the horizontal position was injurious. But we 
found that when there was any slight difference in the 
result, which could be detected only occasionally, the 
leaves which had been fastened closely down suffered 
rather more than those fastened with very long and 






Chap. VI. USE OF SLEEP MOVEMENTS. 287 

thin pins, so as to stand from J to £ inch above the 
cork. This difference in the result, which is in itself 
cnrious as showing what a very slight difference in 
the conditions influences the amount of injury in- 
flicted, may be attributed, as we believe, to the sur- 
rounding warmer air not circulating freely beneath the 
closely pinned leaves and thus slightly warming them. 
This conclusion is supported by some analogous facts 
hereafter to be given. 

We will now describe in detail the experiments 
which were tried. These were troublesome from our 
not being able to predict how much cold the leaves of 
the several species could endure. Many plants had 
every leaf killed, both those which were secured in 
a horizontal position and those which were allowed to 
sleep — that is, to rise up or sink down vertically. 
Others again had not a single leaf in the least in- 
jured, and these had to be re-exposed either for a 
longer time or to a lower temperature. 

Oxalis acetosella. — A very large pot, thickly covered with 
between 300 and 400 leaves, had been kept all winter in the 
greenhouse. Seven leaves were pinned horizontally open, 
and were exposed on March 16th for 2 h. to a clear sky, the 
temperature on the surrounding grass being — 4° C. (24° to 
25° F.). Next morning all seven leaves were found quite 
killed, so were many of the free ones which had previously 
gone to sleep, and about 100 of them, either dead or browned 
and injured, were picked off. Some leaves showed that they 
had been slightly injured by not expanding during the whole 
of the next day, though they afterwards recovered. As all the 
leaves which were pinned open were killed, and only about a 
third or fourth of the others were either killed or injured, we 
had some little evidence that those which were prevented from 
assuming their vertically dependent position suffered most. 

The following night (17th) was clear and almost equally cold 
(— 3° to — 4° O. on the grass), and the pot was again exposed 
but this time for only 30 m. Eight leaves had been pinned out, 



288 MODIFIED CIKCUMNUTATION. Chap. \I 

and in the morning two of them were dead, whilst not a single 
other leaf on the many plants was even injured. 

On the 23rd the pot was exposed for 1 h. 30 m., the tempera- 
ture on the grass being only - 2° C, and not one leaf was 
injured: the pinned open leaves, however, all stood from 
2 to I of an inch above the cork. 

On the 24th the pot was again placed on the ground and 
exposed to a clear sky for between 35 m. and 40 m. By a mis- 
take the thermometer was left on an adjoining sun-dial 3 feet 
high, instead of being placed on the grass ; it recorded 25° to 
26° F. (-33° to - 3-8° C), but when looked at after 1 h. had 
fallen to 22° F. (- 5*5° C); so that the pot was perhaps exposed 
to rather a lower temperature than on the two first occasions. 
Eight leaves had been pinned out, some close to the cork and 
some above it, and on the following morning five of them (i.e. 
63 per cent.) were found killed. By counting a portion of the 
leaves we estimated that about 250 bad been allowed to go to 
sleep, and of these about 20 were killed (i.e. only 8 per cent.), 
and about 30 injured. 

Considering these cases, there can be no doubt that the 
leaves of this Oxalis, when allowed to assume their normal 
vertically dependent position at night, suffer much less from 
frost than those (23 in number) which had their upper surfaces 
exposed to the zenith. 

Oxalis carnosa. — A plant of this Chilian species was exposed 
for 30 m. to a clear sky, the thermometer on the grass standing 
at — 2° C , with some of its leaves pinned open, and not one leaf 
on the whole bushy plant was in the least injured. On the 
16th of March another plant was similarly exposed for 30 m., 
when the temperature on the grass was only a little lower, viz , 
— 3° to - 4° C. Six of the leaves had been pinned open, and 
next morning five of them were found much browned. The 
plant was a large one, and none of the free leaves, which 
were asleep and depended vertically, were browned, excepting 
four very young ones. But three other leaves, though not 
browned, were in a rather flaccid condition, and retained their 
nocturnal position during the whole of the following day. In 
this case it was obvious that the leaves which w-ere exposed hori- 
zontally to the zenith suffered most. This same pot was after- 
wards exposed for 35-^tO m. on a slightly colder night, and 
every leaf, both the pinned open and the free ones, was killed 
It may be added that two pots of O. comiculuta (var. Atro 



Chap. VI. USE OF SLEEP MOVEMENTS. 289 

purpurea) were exposed for 2 h. and 3 h. to a clear sky with the 
temp, on grass — 2° C, and none of the leaves, whether free or 
pinned open, were at all injured. 

Arachis hypogcea. — Some plants in a pot were exposed at night 
for 30 m. to a clear sky, the temperature on the surrounding 
grass being — 2° C, and on two nights afterwards they were again 
exposed to the same temperature, but this time during 1 h. 30 m. 
On neither occasion was a single leaf, whether pinned open or 
free, injured ; and this surprised us much, considering its native 
tropical African home. Two plants were next exposed (March 
16th) for 30 m. to a clear sky, the temperature of the surrounding- 
grass being now lower, viz., between — 3° and — 4° C, and all 
four pinned-open leaves were killed and blackened. These two 
plants bore 22 other and free leaves (excluding some very young 
bud-like ones) and only two of these were killed and three some- 
what injured ; that is, 23 per cent, were either killed or injured, 
whereas all four pinned open leaves were utterly killed. 

On another night two pots with several plants were exposed 
for between 35 m. and 40 m. to a clear sky, and perhaps to a 
rather lower temperature, for a thermometer on a dial, 3 feet 
high, close by stood at - 3'3° to - 3'8° C. In one pot three 
leaves were pinned open, and all were badly injured; of the 
44 free leaves, 26 were injured, that is, 59 per cent. In the 
other pot 3 leaves were pinned open and all were killed; four 
other leaves were prevented from sleeping by narrow strips of 
stiff paper gummed across them, and all were killed ; of 24 free 
leaves, 10 were killed, 2 much injured, and 12 unhurt; that is, 
50 per cent, of the free leaves were either killed or much in- 
jured. Taking the two pots together, we may say that rather 
more than half of the free leaves, which were asleep, were either 
killed or injured, whilst all the ten horizontally extended leaves, 
which had been prevented from going to sleep, were either killed 
or much injured. 

Cassia floribunda. — A bush was exposed at night for 40 m. to 
a clear sky, the temperature on the surrounding grass being 
— 2° C, and not a leaf was injured.* It was again exposed on 



* Cassia Ixvigata was exposed injured. But when C. laevigata 

to a clear sky for 35 ra., and G. was exposed for 1 h., the temp. 

calliantha (a Guiana species) for on the surrounding grass being 

(30 m., the temperature on the between — 3° and — 4° C, everj 

surrounding grass being — 2° C, leaf was killed. 
and neither were in the least 



290 MODIFIED CTRCUMNUTATION. Chap. VI. 

another night for lh., when the temperature of the grass was 
— 4° C. ; and now all the leaves on a large bush, whether pinned 
fiat open or free, were killed, blackened, and shrivelled, with 
the exception of those on one small branch, low down, which 
was very slightly protected by the leaves on the branches 
above. Another tall bush, with four of its large compound 
leaves pinned out horizontally, was afterwards exposed (temp. 
of surrounding grass exactly the same, viz., — 4° C), but only 
for 30 m. On the fo'lowing morning every single leaflet on 
these four leaves was dead, with both their upper and lower 
surfaces completely blackened. Of the many free leaves on the 
bush, only seven were blackened, and of these only a single one 
(which was a younger and more tender leaf than any of the 
pinned ones) had both surfaces of the leaflets blackened. The 
contrast in this latter respect was well shown by a free leaf, which 
stood between two pinned-open ones; for these latter had the 
lower surfaces of their leaflets as black as ink, whilst the inter- 
mediate free leaf, though badly injured, still retained a plain 
tinge of green on the lower surface of the leaflets. This bush 
exhibited in a striking manner the evil effects of the leaves not 
being allowed to assume at night their normal dependent posi- 
tion ; for had they all been prevented from doing so, assuredly 
every single leaf on the bush would have been utterly killed by 
this exposure of only 30 m. The leaves whilst sinking down- 
wards in the evening twist round, so that the upper surface is 
turned inwards, and is thus better protected than the outwardly 
turned lower surface. Nevertheless, it was always the upper 
surface which was more blackened than the lower, whenever 
any difference could be perceived between them ; but whether this 
was due to the cells near the upper surface being more tender, 
or merely to their containing more chlorophyll, we do not know. 
Mdilotus officinalis. — A large pot w r ith many plants, which 
had been kept during the winter in the greenhouse, was exposed 
during 5 h. at night to a slight frost and clear sky. Four 
leaves had been pinned out, and these died after a few days ; 
but so did many of the free leaves. Therefore nothing certain 
could be inferred from this trial, though it indicated that the 
Horizontally extended leaves suffered most. Another large pot 
with many plants was next exposed for 1 h., the temperature on 
the surrounding grass being lower, viz., — 3° to — 4° C. Ten 
leaves had been pinned out, and the result was striking, for 
on the following morning all these were found much injured or 



Chap. VI. USE OF SLEEP MOVEMENTS. 291 

killed, and none of the many free leaves on the several plants 
were at all injured, with the doubtful exception of two or 
three very young ones. 

Melilotus Italica. — Six leaves were pinned out horizontally, 
three with their upper and three with their lower surfaces turned 
to the zenith. The plants were exposed for 5 h. to a clear sky, 
the temperature on ground being about — 1° C. Next morning 
the six pinned-open leaves seemed more injured even than the 
younger and more tender free ones on the same branches. The 
exposure, however, had been too long, for after an interval of 
some days many of the free leaves seemed in almost as bad a 
condition as the pinned-out ones. It was not possible to decide 
whether the leaves with their upper or those with their lower 
surfaces turned to the zenith had suffered most. 

Melilotus suaveoleits. — Some plants with 8 leaves pinned out 
were exposed to a clear sky during 2 h., the temperature on the 
surrounding grass being — 2° C. Next morning 6 out of these 
8 leaves were in a flaccid condition. There were about 150 free 
leaves on the plant, and none of these were injured, except 2 or 3 
very young ones. But after two days, the plants having been 
brought back into the greenhouse, the 6 pinned-out leaves all 
recovered. 

Melilotus Taurica. — Several plants were exposed for 5 h- during 
two nights to a clear sky and slight frost, accompanied by some 
wind; and 5 leaves which had been pinned out suffered more 
than those both above and below on the same branches which 
had gone to sleep. Another pot, which had likewise been kept 
in the greenhouse, was exposed for 35-40 m. to a clear sky, 
the temperature of the surrounding grass being between — 3° and 

— 4° C. Nine leaves had been pinned out, and all of these were 
killed. On the same plants there were 210 free leaves, which 
had been allowed to go to sleep, and of these about 80 were 
killed, i.e. only 38 per cent. 

Melilotus Fef/itjoitrnana. — The plants were exposed to a clear 
sky for 35-40 m. : temperature on surrounding grass — 3° to 

— 4° C. Six leaves had been pinned out so as to stand about 
k inch above the cork, and four had been pinned close to it. 
These 10 leaves were all killed, but the closely pinned ones 
suffered most, as 4 of the 6 which stood above the cork still 
retained small patches of a green colour. A considerable 
number, but not nearly all, of the free leaves, were killed or 
much injured, whereas all the pinned out ones were killed. 



292 MODIFIED CIKCUMNUTATION. Ciiai- VI. 

Melilotus macrorrJiiza. — The plants were exposed in the same 
manner as in the last case. Six leaves had been pinned out 
horizontally, and five of them were killed, that is, 83 per cent. 
We estimated that there were 200 free leaves on the plants, and 
of these about 50 were killed and 20 badly injured, so that abou! 
85 per cent, of the free leaves were killed or injured. 

Lotus aristata. — Six plants were exposed for nearly 5 h. to a 
clear sky ; temperature on surrounding grass - 1'5° C. Four 
leaves had been pinned out horizontally, and 2 of these suffered 
more than those above or below on the same branches, which 
had been allowed to go to sleep. It is rather a remarkable fact 
that some plants of Lotus Jacohosus, an inhabitant of so hot a 
country as the Cape Verde Islands, were exposed one night to a 
clear sky, with the temperature of the surrounding grass — 2° C, 
and on a second night for 30 m. with the temperature of 
the grass between — 3° and — 4° C, and not a single leaf, either 
the pinned-out or free ones, was in the least injured. 

Marsilea quadrifoliata. — A large plant of this species — the 
only Cryptogamic plant known to sleep— with some leaves pinned 
open, was exposed for 1 h. 35 m. to a clear sky, the temperature 
on the surrounding ground being — 2° C, and not a single leaf 
was injured. After an interval of some days the plant was again 
exposed for 1 h. to a clear sky, with the temperature on the 
surrounding ground lower, viz., — 4° C. Six leaves had been 
pinned out horizontally, and all of them were utterly killed. 
The plant had emitted long trailing stems, and these had been 
wrapped round with a blanket, so as to protect them from the 
frozen ground and from radiation; but a very large number 
of leaves were left freely exposed, which had gone to sleep, 
and of these only 12 were killed. After another interval, th6 
plant, with 9 leaves pinned out, was again exposed for 1 h., the 
temperature on the ground being again — 4° C. Six of the leaves 
were killed, and one which did not at first appear injured after- 
wards became streaked with brown. The trailing branches, which 
rested on the frozen ground, had one-half or three-quarters of their 
leaves killed, but of the many other leaves on the plant, which 
alone could be fairly compared with the pinned-out ones, none 
appeared at first sight to have been killed, but on careful search 
12 were found in this state. After another interval, the plant 
with 9 leaves pinned out, was exposed for 35-40 m. to a clear 
sky and to nearly the same, or perhaps a rather lower, tempera- 
ture (for the thermometer by an accident had been left on a 



Chap. VI. USE OF SLEEP MOVEMENTS. 293 

Bun-dial close by), and. 8 of these leaves were killed. Of the free 
leaves (those on the trailing branches not being considered), a 
good many were killed, but their number, compared with the 
uninjured ones, was small. Finally, taking the three trials 
together, 24 leaves, extended horizontally, were exposed to the 
zenith and to unobstructed radiation, and of these 20 were 
killed and 1 injured ; whilst a relatively very small proportion 
of the leaves, which had been allowed to go to sleep with their 
leaflets vertically dependent, were killed or injured. 

The cotyledons of several plants were prepared for trial, but 
the weather was mild and we succeeded only in a single instance 
in having seedlings of the proper age on nights which were 
clear and cold. The cotyledons of 6 seedlings of Mimosa pudica 
were fastened open on cork, and were thus exposed for 1 h. 45 m, 
to a clear sky, with the temperature on the surrounding ground 
at 29° F. ; of these, 3 were killed. Two other seedlings, after 
their cotyledons had risen up and had closed together, were 
bent over and fastened so that they stood horizontally, with the 
lower surface of one cotyledon fully exposed to the zenith, and 
both were killed. Therefore of the 8 seedlings thus tried 5, or 
more than half, were killed. Seven other seedlings, with their 
cotyledons in their normal nocturnal position, viz., vertical and 
closed, were exposed at the same time, and of these only 2 were 
killed.* Hence it appears, as far as these few trials tell anything, 
that the vertical position at night of the cotyledons of Mimosa 
pudica protects them to a certain degree from the evil effects of 
radiation and cold. 

Concluding Remarks on the Radiation from Leaves 
at Night. — We exposed on two occasions during the 
summer to a clear sky several pinned-open leaflets 
of Trifolium pratense, which naturally rise at night, 
and of Oxalis purpurea, which naturally sink at night 
(the plants growing out of doors), and looked at 



* We were surprised that It may be added that seedlings of 

young seedlings of so tropical a the Indian Cassia pubescens were 

plant as Mimosa pudica were able exposed fur 1 b.. 30 m. to a clear 

to resist, as well as they did, ex- sky, with the temp, on the sur- 

posure for ] hr. 45 m. to a clear rounding ground at — 2° C, and 

fciky, the temperature on the sur- they were not in the least injured 
rounding ground being 29° P. 



294 MODIFIED CIBCUMNUTATIOy. Chat. VI 

theni early on several successive mornings, after they 
had assumed their diurnal positions. The difference 
in the amount of clew on the pinned-open leaflets 
and on those which had gone to sleep was generally 
conspicuous ; the latter beiug sometimes absolutely 
dry, whilst the leaflets which had been horizontal 
were coated Avith large beads of dew. This shows how 
much cooler the leaflets fully exposed to the zenith 
must have become, than those which stood almost 
vertically, either upwards or downwards, during the 
nio-ht. 

From the several cases above given, there can be no 
doubt that the position of the leaves at night affects 
their temperature through radiation to such a degree, 
that when exposed to a clear sky during a frost, it is a 
question of life and death. We may therefore admit 
as highly probable, seeing that their nocturnal posi- 
tion is so well adapted to lessen radiation, that the 
object gained by their often complicated sleep move- 
ments, is to lessen the degree to which they are 
chilled at night. It should be kept in mind that 
it is especially the upper surface which is thus pro- 
tected, as it is never directed towards the zenith, and 
is often brought into close contact with the upper 
surface of an opposite leaf or leaflet. 

We failed to obtain sufficient evidence, whether 
the better protection of the upper surface has been 
gained from its being more easily injured than the 
lower surface, or from its injury being a greater evi] 
to the plant. That there is some difference in consti- 
tution between the two surfaces is shown by the follow- 
ing cases. Cassia floribunda was exposed to a clear sky 
on a sharp frosty night, and several leaflets which 
had assumed their nocturnal dependent position with 
their lower surfaces turned outwards so as to be 



Chap. VI. USE OF SLEEP MOVEMENTS. 295 

exposed obliquely to the zenith, nevertheless had these 
lower surfaces less blackened than the upper surfaces 
which were turned inwards and were in close contact 
with those of the opposite leaflets. Again, a pot 
full of plants of Trifolium resupinatum, which had 
been kept in a warm room for three days, was turned 
out of doors (Sept. 21st) on a clear and almost frosty 
night. Next morning ten of the terminal leaflets were 
examined as opaque objects under the microscope. 
These leaflets, in going to sleep, either turn vertically 
upwards, or more commonly bend a little over the 
lateral leaflets, so that their lower surfaces are more 
exposed to the zenith than their upper surfaces. 
Nevertheless, six of these ten leaflets were distinctly 
yellower on the upper than on the lower and more 
exposed surface. In the remaining four, the result 
was not so plain, but certainly whatever difference 
there was leaned to the side of the upper surface 
having suffered most. 

It has been stated that some of the leaflets experi- 
mented on were fastened close to the cork, and others 
at a height of from J to f of an inch above it ; and 
that whenever, after exposure to a frost, any difference 
could be detected in their states, the closely pinned 
ones had suffered most. We attributed this difference 
to the air, not cooled by radiation, having been pre- 
vented from circulating freely beneath the closely 
pinned leaflets. That there was really a difference in 
the temperature of leaves treated in these two dif- 
ferent methods, was plainly shown on one occasion ; 
for after the exposure of a pot with plants of Melilotus 
dentaia for 2 h. to a clear sky (the temperature on the 
surrounding grass being — 2° C), it was manifest that 
more dew had congealed into hoar-frost on the closely 
pinned leaflets, than on those which stood horizontally 



296 MODIFIED CIKCUMNUTATION. Chap. VL 

a little above the cork. Again, the tips of some few 
leaflets, which had been pinned close to the cork, pro- 
ectecl a little beyond the edge, so that the air could 
circulate freely round them. This occurred with six 
leaflets of Oi'alis acetosella, and their tips certainly 
suffered rather less than the rest of the same leaflets ; 
lor on the following morning they were still slightly 
green. The same result followed, even still more 
clearly, in two cases with leaflets of Melilotus officinalis 
which projected a little beyond the cork; and in two 
other cases some leaflets which were pinned close to 
the cork were injured, whilst other free leaflets on 
the same leaves, which had not space to rotate and 
assume their proper vertical position, were not at all 
injured. 

Another analogous fact deserves notice : we observed 
on several occasions that a greater number of free 
leaves were injured on the branches which had been 
kept motionless by some of their leaves having been 
pinned to the corks, than on the other branches. This 
was conspicuously the case with those of Melilotus 
Peiitjnerreana, but the injured leaves in this instance 
were not actually counted. With Aracliis lujpogma, a 
young plant with 7 stems bore 22 free leaves, and of 
these 5 were injured by the frost, all of which were on 
two stems, bearing four leaves pinned to the cork- 
supports. With Oxalis carnosa, 7 free leaves were 
injured, and every one of them belonged to a cluster 
of leaves, some of which had been pinned to the cork. 
We could account for these cases only by supposing 
that the branches which were quite free had been 
slightly waved about by the wind, and that their 
leaves had thus been a little warmed by the sur- 
rounding warmer air. If we hold our hands motion 
less before a hot fire, and then wave them about, we 






Chap. VI. SLEEP OF COTYLEDONS. 297 

immediately feel relief; and this is evidently an 
analogous, though reversed, ease. These several facts 
— in relation to leaves pinned close to or a little above 
the cork-supports — to their tips projecting beyond it — 
and to the leaves on branches kept motionless — seem 
to us curious, as showing how a difference, apparently 
trifling, may determine the greater or less injury of 
the leaves. We may even infer as probable that the 
less or greater destruction during a frost of the leaves 
on a plant which does not sleep, may often depend on 
the greater or less degree of flexibility of their petioles 
and of the branches which bear them. 

Nyctitropic or Sleep Movements of Cotyledons. 

We now come to the descriptive part of our work, 
and will begin with cotyledons, passing on to leaves 
in the next chapter. We have met with only two 
brief notices of cotyledons sleeping. Hofmeister,* 
after stating that the cotyledons of all the observed 
seedlings of the Caryophyllese (Alsineae and Silenese) 
bend upwards at night (but to what angle he does not 
state), remarks that those of Stellaria media rise up so 
as to touch one another ; they may therefore safely be 
said to sleep. Secondly, according to Ramey, f the 
cotyledons of Mimosa joudica and of Cliantlius Dam- 
pier i rise up almost vertically at night and approach 
each other closely. It has been shown in a previous 
chapter that the cotyledons of a large number of 
plants bend a little upwards at night, and we here 
have to meet the difficult question at what inclination 
may they be said to sleep? According to the view 
ahich we maintain, no movement deserves to be called 



* ' Die Lehre von der Pflnnzenzelle,' 18G7, p. 327. 
t * Adansoiiia' March 10th, 18G9. 



k 298 MODIFIED OIRCUMNUTATION. Chap. VI 

nyctitropic, unless it has been acquired for the sake of 
lessening radiation ; but this could be discovered only 
by a long series of experiments, showing that the 
leaves of each species suffered from this cause, if pre- 
vented from sleeping. We must therefore take an 
arbitrary limit. If a cotyledon or leaf is inclined at 
60° above or beneath the horizon, it exposes to the 
zenith about one-half of its area ; consequently the 
intensity of its radiation will be lessened by about 
half, compared with what it would have been if the 
cotyledon or leaf had remained horizontal. This 
degree of diminution certainly would make a great 
difference to a plant having a tender constitution. 
We will therefore speak of a cotyledon and hereafter 
of a leaf as sleeping, only when it rises at night to 
an angle of about 60°, or to a still higher angle, above 
the horizon, or sinks beneath it to the same amount. 
Not but that a lesser diminution of radiation may be 
advantageous to a plant, as in the case of Datura 
stramonium, the cotyledons of which rose from 31° at 
noon to 55° at night above the horizon. The Swedish 
turnip may profit by the area of its leaves being 
reduced at night by about 30 per cent., as estimated 
by M r. A. S. Wilson ; though in this case the angle 
through which the leaves rose was not observed. On 
the other hand, when the angular rise of cotyledons or 
of leaves is small, such as less than 30°, the diminution 
of radiation is so slight that it probably is of no sig- 
nificance to the plant in relation to radiation. For 
instance, the cotyledons of Geranium Ihericum rose at 
night to 27° above the horizon, and this would lessen 
radiation by only 11 per cent. : those of Linum Beren- 
dieri rose to 33°, and this would lessen radiation by 
16 per cent. 

There are, however, some other sources of doubt witD 



Chap. VI. SLEEP OF COTYLEDONS. 299 

respect to the sleep of cotyledons. In certain cases, 
the cotyledons whilst young diverge during the day to 
only a very moderate extent, so that a small rise at 
night, which we know occurs with the cotyledons of 
many plants, would necessarily cause them to assume 
a vertical or nearly vertical position at night ; and in 
this case it would be rash to infer that the movement 
was effected for any special purpose. On this account 
we hesitated long whether we should introduce several 
Cucurbitaceous ,plants into the following list ; but from 
reasons, presently to be given, we thought that they 
had better be at least temporarily included. . This 
same source of doubt applies in some few other cases ; 
for at the commencement of our observations we did 
not always attend sufficiently to whether the cotyle- 
dons stood nearly horizontally in the middle of the day. 
With several seedlings, the cotyledons assume a highly 
inclined position at night during so short a period of 
their life, that a doubt naturally arises whether this 
can be of any service to the plant. Nevertheless, in 
most of the cases given in the following list, the coty- 
ledons may be as certainly said to sleep as may the 
leaves of any plant. In two cases, namely, with the 
cabbage and radish, the cotyledons of which rise almost 
vertically during the few first nights of their life, it 
was ascertained by placing young seedlings in the 
klinostat, that the upward movement was not due to 
apogeotropism. 

The names of the plants, the cotyledons of which 
stand at night at an angle of at least 60 D with the 
horizon, are arranged in the appended list on the same 
system as previously followed. The numbers of the 
Families, and with the LeguminosaB the numbers of 
the Tribes, have been added to show how widely 
the plants in question are distributed throughout the 



MODIFIED CIKCUMNUTATION. 



Chap. VL 



dicotvledonous series. A few remarks will have to 
be made about many of the plants in the list. In 
doing so, it will be convenient not to follow strictly 
any systematic order, but to treat of the Oxalidae 
and the Legummosae at the close ; for in these 
two Families the cotyledons are generally provided 
with a pulvinus, and their movements endure for a 
much longer time than those of the other plants in 
the list. 



List of Seedling Plants, the cotyledons of which rise or sink at 
night to an angle of at least 60° above or beneath the horizon. 



Brassica oleracea. Cruciferse (Fam. 

14). 
napus (as we are informed 

by Prof. Pfefter). 
Eaphauus sativus. Cruciferse. 
Githago segetum. Caryophylleae 

(Fam. 26). 
Stellaria media (according to Hof- 

meister, as quoted). Caryophyl- 
leae. 
Anoda Wrightii. Malvaceae (Fam. 

36). 
Gossypium (var. Nankin cotton). 

Malvaceae. 
Oxalis rosea. Oxalidae (Fam. 41). 

floribunda. 

articulata. 

Valdiviana. 

sensitiva. 

Geranium rotundifolium 



Gera- 
Legu- 



niaceae (Fam. 47). 
Trifolium subterraneum. 

minosae (Fam. 75, Tribe 3). 

strictum. 

leucanthemum. 

Lotus ornithopopoides. Legumi- 

nosae (Tribe 4). 
peregrinus. 



Jacobaeus, 

Clianthus Dampieri. Legumi- 

nosae (Tribe 5) — according to M. 

Ramey. 
Smithia sensitiva. Leguminosa> 

(Tribe 6). 
Haematoxvlon Campechianum. Le- 



guminosae (Tribe 13) — accord- 
ing to Mr. K. I. Lynch. 

Cassia mimosoides. Leguminosae 
(Tribe 14). 

glauca. 

florida. 

corymbosa. 

pubeseens. 

tora. 

neglecta. 

3 other Brazilian unnamed 



?)• 



species. 
Bauhinia (sp. 

(Tribe 15). 
Neptunia oleracea. 

(Tribe 20). 
Mimosa pudica. 

(Tribe 21). 

albida. 

Cucurbita ovifera. 



Leguminosa 1 
Leguminosae 
Leguminosae 

Cucurbitaceae 



(Fam. 106). 

aurantia. 

Lagenaria vulgaris. Cucurbitaceae. 
Cucumis dudaim. Cucurbitaceae. 
Apium petroselinum. Umbelliferae 

(Fam. 113). 

graveolens. 

Lactuca scariola. Compositae (Fam. 

122). 
Helianthus annuus (?). Compositae, 
Ipomcea cserulea. Convoh ulacea 

(Fam. 151). 

purpurea. 

bona-nox. 

coccinea. 



Chap. VI. SLEEP OF COTYLEDONS. 301 



List of Seedling Plants (continued). 

Mirabilis longiflora. 

Beta vulgaris. Polygoneae (Fanci. 

179). 
Amaranthus caudatus. Amaran- 

thacese (Fam. 18u). 
Cannabis sativa (?). Cannabineae 

(Fam. 195). 



Rolanum lycopersicum. Solanese 

(Fam. 157). 
Mimulus, (sp. ?) Scrophularinese 

(Fam. 159) — from information 

given us by Prof. Pfeffer. 
Mirabilis jalapa. Nyctagineaj 

(Fam. 177). 



Brassica olerarea (Crucifcrse). —It was shown in the first chapter 
that the cotyledons of the common cabbage rise in the evening 
and stand vertically np at night with their petioles in contact. 
But as the two cotyledons are of unequal height, they frequently 
interfere a little with each other's movements, the shorter one 
often not standing quite vertically. They awake early in the 
morning; thus at 6.45 a.m. on Nov. 27th, whilst it was still 
dark, the cotyledons, which had been vertical and in contact on 
the previous evening, were reflexed, and thus presented a very 
different appearance. It should be borne in mind that seedlings 
in germinating at the proper season, would not be subjected to 
darkness at this hour in the morning. The above amount of 
movement of the cotyledons is only temporary, lasting with plants 
kept in a warm greenhouse from four to six days ; how long it 
would last with seedlings growing out of doors we do not know. 

Raphanus sativus.—lri the middle of the day the blades of 
the cotyledons of 10 seedlings stood at right angles to their 
hypocotyls, with their petioles a little divergent ; at night the 
blades stood vertically, with their bases in contact and with 
their petioles parallel. Next morning, at 6.45 a.m., whilst it 
was still dark, the blades were horizontal. On the following- 
night they were 'much raised, but hardly stood sufficiently ver- 
tical to be said to be asleep, and so it was in a still less degree 
on the third night. Therefore the cotyledons of this plant (kept 
in the greenhouse) go to sleep for even a shorter time than 
those of the cabbage. Similar observations were made, but only 
during a single day and night, on 13 other seedlings likewise 
raised in the greenhouse, with the same result. 

The petioles of the cotyledons of 11 young seedlings of 
Sinapis nigra were slightly divergent at noon, and the blades 
stood at right angles to the hypocotyls ; at night the petioles 
were in close contact, and the blades considerably raised, 
with their bases in contact, but only a few stood sufficiently 
upright to be called asleep. On the following morning, 



302 MODIFIED CIKCUMNUTATION. Chat. VI. 

the petioles diverged before it was light. The hypocotyl in 
slightly sensitive, so that if rubbed with a needle it bends 
towards the rubbed side. In the case of Lepidium sativum, the 
petioles of the cotyledons of young seedlings diverge during 
the day and converge so as to touch each other during the 
night, by which means the bases of the tripartite blades are 
brought into contact ; but the blades are so little raised that 
they cannot be said to sleep. The cotyledons of several other 
cruciferous plants were observed, but they did not rise sufficiently 
during the night to be said to sleep. 

Githago segetum (Caryophyllese). — On the first day after the 
cotyledons had burst through the seed-coats, they stood at noon 
at an angle of 75° above the horizon ; at night they moved 
upwards, each through an angle of 15° so as to stand quite 
vertical and in contact with one another. On the second day 
they stood at noon at 59° above the horizon, and again at 
night were completely closed, each having risen 31°. On the 
fourth day the cotyledons did not quite close at night. The 
first and succeeding pairs of young true leaves behaved in 
exactly the same manner. We think that the movement in this 
case may be called nycti tropic, though the angle passed through 
was small. The cotyledons are very sensitive to light and will 
not expand if exposed to an extremely dim one. 

Anoda Wrightii (Malvacese). — The cotyledons whilst moderately 
young, and only from -2 to *3 inch in diameter, sink in the 
evening from their mid-day horizontal position to about 35° 
beneath the horizon. But when the same seedlings were older 
and had produced small true leaves, the almost orbicular 
cotyledons, now *55 inch in diameter, moved vertically downwards 
at night. This fact made us suspect that their sinking might 
be due merely to their weight ; but they were not in the least 
flaccid, and when lifted up sprang back through elasticity into 
their former dependent position. A pot with some old seedlings 
was turned upside down in the afternoon, before the noc- 
turnal fall had commenced, and at night they assumed in op- 
position to their own weight (and to any geotropic action) an 
upwardly directed vertical position. When pots were thus 
reversed, after the evening fall had already commenced, the 
sinking movement appeared to be somewhat disturbed ; but all 
their movements were occasionally variable without any apparent 
cause. This latter fact, as well as that of the young cotyledons 
not sinking nearly so much as the older ones, deserves notice. 



Chai-. VI. SLEEP OF COTYLEDONS. 303 

Although the movement of the cotyledons endured for a long 
time, no pulvinus was exteriorly visible; but their growth 
continued for a long time. The cotyledons appear to be only 
slightly heliotropic, though the hypocotyl is strongly so. 

Gassy pium arboi -turn (?) (var. Nankin cotton) (Malvaceae). — The 
cotyledons behave in nearly the same manner as those of the 
Anoda. On June 15th the cotyledons of two seedlings were 
'65 inch in length (measured along the midrib) and stood hori- 
zontally at noon ; at 10 p.m. they occupied the same position 
and had not fallen at all. On June 23rd, the cotyledons of one 
of these seedlings were 11 inch in length, and by 10 p.m. they 
had fallen from a horizontal position to 62° beneath the horizon. 
The cotyledons of the other seedling were 1*3 inch in length, arid 
a minute true leaf had been formed; they had fallen at 10 p.m. 
to 70° beneath the horizon. On June 25th, the true leaf of this 
latter seedling was *9 inch in length, and the cotyledons occu- 
pied nearly the same position at night. By July 9th the cotyle- 
dons appeared very old and showed signs of withering; but they 
stood at noon almost horizontally, and at 10 p.m. hung down 
vertically. 

Gossypium herbiceum. — It is remarkable that the cotyledons of 
this species behave differently from those of the last. They were 
observed during 6 weeks from their first development until 
they had grown to a very large size (still appearing fresh and 
green), viz. 2£ inches in breadth. At this age a true leaf had 
been formed, which with its petiole was 2 inches long. During 
the whole of these 6 weeks the cotyledons did not sink at night ; 
yet when old their weight was considerable and they were borne 
by much elongated petioles. Seedlings raised from some seed 
sent us from Naples, behaved in the same manner ; as did those 
of a kind cultivated in Alabama and of the Sea-island cotton. 
To what species these three latter forms belong we do not know. 
"We could not make out in the case of the Naples cotton, that 
the position of the cotyledons at night was influenced by the 
soil being more or less dry ; care being taken that they were 
not rendered flaccid by being too dry. The weight of the large 
cotyledons ot the Alabama and Sea-island kinds caused them to 
hang somewhat downwards, when the pots in which they grew 
were left for a time upside down. It should, however, be 
observed that these three kinds were raised in the middle of 
the winter, which sometimes greatly interferes with the proper 
nyctitropic movements of leaves and cotyledons. 



304 MODIFIED CIRCUMNUTATIOtf. Chap. VI. 

Cucurbitacece. — The cotyledons of Cucurbita aurantia and ovi- 
fera, and of Lagenaria vulgaris, stand from the 1st to the 3rd day 
of their life at about 60° above the horizon, and at night rise up 
so as to become vertical and in close contact with one another. 
"With Cucumis dudaim- they stood at noon at 45° above the hori- 
zon, and closed at night. The tips of the cotyledons of all these 
species are, however, reflexed, so that this part is fully exposed 
to the zenith at night ; and this fact is opposed to the belief 
that the movement is of the same nature as that of sleeping 
plants. After the first two or three days the cotyledons 
diverge more during the day and cease to close at night. 
Those of Trichosanthes anguina are somewhat thick and fleshy, 
and did not rise at night ; and they could perhaps hardly be 
expected to do so. On the other hand, those of Acanthosicyos 
horrida * present nothing in their appearance opposed to their 
moving at night in the same manner as the preceding species ; 
yet they did not rise up in any plain manner. This fact leads 
to the belief that the nocturnal movements of the above-named 
species has been acquired for some special purpose, which may 
be to protect the young plumule from radiation, by the close 
contact of the whole basal portion of the two cotyledons. 

Geranium rotundifolium (Geraniacere). — A single seedling came 
up accidentally in a pot, and its cotyledons were observed to 
bend perpendicularly downwards during several successive 
nights, having been horizontal at noon. It grew into a fine 
plant but died before flowering : it was sent to Kew and pro- 
nounced to be certainly a Geranium, and in all probability the 
above-named species. This case is remarkable because the 
cotyledons of G. cinereurn, Endressii, Ibericum, JRicJiardsoni, and 
subcaultscens were observed during some weeks in the winter, 
and they did not sink, whilst those of G. Ibricum rose 27° at 
night. 

Apium petroselinum (Umbelliferas). — A seedling had its coty- 
ledons (Nov. 22nd) almost fully expanded during the day ; by 
8.30 p.m. they had risen considerably, and at 10.30 p.m. were 
almost closed, their tips being only -3-^0 of an inch apart. Ou 
the following morning (23rd) the tips were T 5 ^ of an inch apart, 



* This plant, from Dammara climber ; it 1ms been described 

Land in S. Africa, is remarkable in 'Transact. Linn. Soc.,' xxvii 

from being the ono known mem- p. 30. 
ber of the Family which is not a 



Chap. VI. SLEEP OF COTYLEDONS. 305 

or more than seven times as much. On the next night the 
cotyledons occupied nearly the same position as before. On the 
morning of the 24th they stood horizontally, and at night were 
60° above the horizon ; and so it was on the night of the 25th. 
But four days afterwards (on the 29th), when the seedlings 
were a week old, the cotyledons had ceased to rise at night to 
any plain degree. 

Apium graveolens. — The cotyledons at noon were horizontal, 
and at 10 p.m. stood at an angle of 61° above the horizon. 

Lactuca scariula (Compositse). — The cotyledons whilst young 
stood sub-horizontally during the day, and at night rose so as 
to be almost vertical, and some were quite vertical and closed ; 
but this movement ceased when they had grown old and large, 
after an interval of 11 days. 

Helianthus annuus (Composite). — This case is rather doubtful ; 
the cotyledons rise at night, and on one occasion they stood at 
73° above the horizon, so that they might then be said to have 
been asleep. 

Ipomcea ccerulea vel Pharhitis nil (Convolvulacese). — The coty- 
ledons behave in nearly the same manner as those of the Anoda 
and Nankin cotton, and like them grow to a large size. Whilst 
young and small, so that their blades were from *5 to *6 of an 
inch in length, measured along the middle to the base of the 
central notch, they remained horizontal both during the middle 
of the day and at night. As they increased in size they began 
to sink more and more in the evening and early night ; and 
when they had grown to a length (measured in the above 
manner) of from 1 to 1*25 inch, they sank between 55° and 70° 
beneath the horizon. They acted, however, in this manner only 
when they had been well illuminated during the day. Never- 
theless, the cotyledons have little or no power of bending 
towards a lateral light, although the hypocotyl is strongly helio- 
tropic. They are not provided with a pulvinus, but continue 
to grow for a long time. 

Ipomcea purpurea (vel Pharhitis Jiispidd). — The cotyledons 
behave in all respects like those of /. ccerulea. A seedling with 
cotyledons "75 inch in length (measured as before) and 1-65 
inch in breadth, having a small true leaf developed, was placed 
at 5.30 p.m. on a klinostat in a darkened box, so that neither 
weight nor geotropism could act on them. At 10 p.m. one coty- 
ledon stood at 77° and the other at 82° beneath the horizon. 
Before being placed in the klinostat they stood at 15° and 29° 



306 MODIFIED CIRCUMNUTATION. Chap. VL 

beneath the horizeu. The nocturnal position depends chiefly 
on the curvature of the petiole close to the blade, but the whole 
petiole becomes slightly curved downwards. It deserves notice 
that seedlings of this and the last-named species were raised at 
the end of February and another lot in the middle of March, 
and the cotyledons in neither case exhibited any nyctitropic 
movement. 

Jpomoca bona-nox. — The cotyledons after a few days grow to 
an enormous size, those on a young seedling being 3± inches 
in breadth. They were extended horizontally at noon, and at 
10 p.m. stood at 63° beneath the horizon. Five days after- 
wards they were 4| inches in breadth, and at night one stood at 
64° and the other 48° beneath the horizon. Though the blades 
are thin, yet from their great, size and from the petioles being 
long, we imagined that their depression at night might be 
determined by their weight ; but when the pot was laid hori- 
zontally,, they became curved towards the hypocotyl, which 
movement could not have been in the least aided by their 
weight, at the same time they were somewhat twisted upwards 
through apogeotropism. . Nevertheless, the weight of the coty- 
ledons is so far influential, that when on another night the pot 
was turned upside down, they were unable to rise and thus to 
assume their proper nocturnal position. 

Ipom'ta cocci nea. — The cotyledons whilst young do not sink 
at night, but when grown a little older, but still only *4 inch in 
length (measured as before) and '82 in breadth, they became 
greatly depressed. In one case they were horizontal at noon, 
and at 10 p.m. one of them stood at 64° and the other at 47° 
beneath the horizon. The blades are thin, and the petioles, 
which become much curved down at night, are short, so that 
here weight can hardly have produced any effect. With all the 
above species of Ipomcea, when the two cotyledons on the same 
seedling were unequally depressed at night, this seemed to 
depend on the position which they had held during the day 
with reference to the light. 

Solai i um lycopersicum (Solaneas). — The cotyledons rise so 
much at night as to come nearly in contact. Those of S. pal in a- 
car/thum were horizontal at noon, and by 10 p.m., had risen only 
'27° 30' ; but on the following morning before it was light they 
stood at 59° above the horizon, and in the afternoon of the same 
d;iy were again horizontal. The behaviour of the cotyledons of 
this latter species seems, therefore, to be anomalous. 



Chap. VI. SLEEP OF COTYLEDONS. 307 

Mirabilis jalapa and Ion gi flora (Nyctagineae).— The cotyledons, 
which are of unequal size, stand horizontally during the middle 
of the day, and at night rise up vertically and come into close 
coutact with one another. But this movement with M. longijiora 
lasted for only the three first nights. 

Beta vulgaris (Polygoneae). — A large number of seedlings were 
observed on three occasions. During the day the cotyledons 
sometimes stood sub-horizontally, but more commonly at an 
angle of about 50° above the horizon, and for the first two or 
three nights they rose up vertically so as to be completely 
closed. During the succeeding one or two nights they rose 
only a Lttle, and afterwards hardly at all. 

Amaranthus caudatus (Amaranthacese). — At noon the coty- 
ledons of many seedlings, which had just germinated, stood at 
about 45° above the horizon, and at 10.15 p.m. some were nearly 
and others quite closed. On the following morning they were 
again well expanded or open. 

Cannabis sativa (Cannabineae). — We are very doubtful whether 
this plant ought to be here included. The cotyledons of a large 
number of seedlings, after being well illuminated during the 
day, were curved downwards at night, so that the tips of some 
pointed directly to the ground, but the basal part did not appear 
to be at all depressed. On the following morning they were 
again flat and horizontal. The cotyledons of many other seed- 
lings were at the same time not in any way affected. Therefore 
this case seems very different from that of ordinary sleep, and 
probably comes under the head of epinasty, as is the case with 
the leaves of this plant according to Kraus. The cotyledons are 
heliotropic, and so is the hypocotyl in a still stronger degree. 

Oxalis. — We now come to cotyledons provided with a pulvinus, 
all of which are remarkable from the continuance of the nocturnal 
movements during several days or even weeks, and apparently 
after growth has ceased. The cotyledons of 0. rosea, floribunda 
and articulata sink vertically down at night and clasp the upper 
part of the hypocotyl. Those of 0. Vahiiviana and sensitiva, on 
the contrary, rise vertically up, so that their upper surfaces come 
into close contact; and after the young leaves are developed these 
are clasped by the cotyledons. As in the daytime they stand hori- 
zontally, or are even a little deflected beneath the horizon, they 
move in the evening through an angle of at least 90°. Theii 
complicated circumnutating movements during the day have 



308 MODIFIED CIRCUMNUTATION. Chap. VI 

been described in the first chapter. The experiment was a 
superfluous one, but pots with seedlings of 0. rosea and floribunda 
were turned upside down, as soon as the cotyledons began to 
show any signs of sleep, and this made no difference in their 
movements. 

Leguminosce. — It may be seen in our list that the cotyledons 
of several species in nice genera, widely distributed through- 
out the Family, sleep at night ; and this probably is the case 
with many others. The cotyledons of all these species are pro- 
dded with a pulvinus ; and the movement in all is continued 
during many days or weeks. In Cassia the cotyledons of the 
ten species in the list rise up vertically at night and come 
into close contact with one another. We observed that those 
of G. florida opened in the morning rather later than those of 
G. glauca and pubesce?is. The movement is exactly the same 
in C. mimosoides as in the other species, though its subsequently 
developed leaves sleep in a different manner. The cotyledons 
of an eleventh species, namely, 0. nodosa, are thick and fleshy, 
and do not rise up at night. The circumnutation of the coty- 
ledons during the day of G. tora has been described in the first 
chapter. Although the cotyledons of Smithia sensitiva rose from 
a horizontal position in the middle of the day to a vertical one 
at night, those of S. Pfundii, which are thick and fleshy, did not 
sleep. When Mimosa pudica and albida have been kept at a 
sufficiently high temperature during the day, the cotyledons 
come into close contact at night ; otherwise they merely rise up 
almost vertically. The circumnutation of those of M. pudica 
has been described. The cotyledons of a Bauhinia from St. 
Catharina in Brazil stood during the day at an angle of about 
5L»° above the horizon, and at night rose to 77°; but it is pro- 
bable that they would have closed completely, if the seedlings 
had been kept in a warmer place. 

Lotus. — In three species of Lotus the cotyledons were observed 
to sleep. Those of L. Jacobceus present the singular case of not 
rising at night in any conspicuous manner for the first 5 or 
6 days of their life, and the pulvinus is not well developed at 
this period. Afterwards the sleeping movement is well dis- 
played, though to a variable degree, and is long continued. 
We shall hereafter meet with a nearly parallel case with the 
leaves of Sida rhombifo 7 <a. The cotyledons of L. Gebelii are 
only slightly raised at night, and differ much in this respec' 
from the three species in our list. 



Chap VI. SLEEP OF COTYLEDONS. 309 

Trifulium. — The germination of 21 species was observed. In 
most of them the cotyledons rise hardly at all, .or only slightly, 
at night ; but those of T. glomeratum, striatum and incm natum 
rose from 45° to 55° above the horizon. With T. subterraneum, 
kucanthemum and striatum, they stood up vertically ; and with 
T. strictum the rising movement is accompanied, as we shall see, 
by another movement, which makes us believe that the rising 
is truly nyctitropio. We did not carefully examine the coty- 
ledons of all the species for a pulvinus, but this organ was 
distinctly present in those of T. subterraneum and strictum ; whilst 
there was no trace of a pulvinus in some species, for instance, in 
T. resupinatum, the cotyledons of which do not rise at night. 

Trifolium subterraneum. — The blades of the cotyledons on the 
first day after germination (Nov. 21st) were not fully expanded, 
being inclined at about 35° above the horizon; at night they 
rose to about 75°. Two days afterwards the blades at noon 
were horizontal, with the petioles highly inclined upwards; 
and it is remarkable that the nocturnal movement is almost 
wholly confined to the blades, being effected by the pulvinus at 
their bases ; whilst the petioles retain day and night nearly the 
same inclination. On this night (Nov. 23rd), and for some few 
succeeding nights, the blades rose from a horizontal into a 
vertical position, and then became bowed inwards at about an 
average angle of 10° ; so that they had passed through an angle 
of 100°. Their tips now almost touched one another, their 
bases being slightly divergent. The two blades thus formed 
a highly inclined roof over the axis of the seedling. This 
movement is the same as that of the terminal leaflet of the 
tripartite leaves of many species of Trifolium. After an interval 
of 8 days (Nov. 29th) the blades were horizontal during the 
day, and vertical at night, and now they were no longer bowed 
inwards. They continued to move in the same manner for the 
following two months, by which time they had increased greatly 
in size, their petioles being no less than * 8 of an inch in length, 
and two true leaves had by this time been developed. 

Trifulium strictum. — On the first day after germination the 
cotyledons, which are provided with a pulvinus, stood at noon 
horizontally, and at night rose to only about 45° above the 
horizon. Four days afterwards the seedlings were again ob- 
served at night, and now the blades stood vertically and were 
in contact, excepting the tips, which were much deflexed, so 
that they faced the zenith. At this age the petioles are curved 



310 



MODIFIED CIKCUMNUTATION. 



Chap. VI. 



upwards, and at night, when the bases of the blades are in con- 
tact, the two petioles together form a vertical ring surrounding 
the plumule. The cotyledons continued to act in Dearly the same 
manner for 8 or 10 days from the period of germination ; but 
the petioles had by this time become straight and had increased 
much in length. After from 12 to 14 days the first simple true 
leaf was formed, and during the ensuing fortnight a remarkable 
movement was repeatedly observed. At I. (Fig. 125) we have 
a sketch, made in the middle of the day, of a seedling about 
a fortnight old. The two cotyledons, of which Re is the 
light, and Lc the left one, stand directly opposite one another, 




Trifolium strictum ; diurnal and nocturnal positions of the two cotyledons 
and of the first leaf. I. Seedling viewed obliquely from above, during: 
the day: Be, right cotyledon; Lc, left cotyledon; F, first true leaf. 
II. A rather younger seedling, viewed at night: Be, right cotyledon 
raised, but its position not otherwise changed ; Lc, left cotyledon raised 
and laterally twisted; F, first leaf raised and twisted so as to face the 
left twisted cotyledon. III. Same seedling viewed at night from the 
opposite side. The back of the first leaf, F, is here shown instead of 
the front, as in II. 

and the first true leaf (F) projects at right angles to them. At 
night (see II. and III.) the right cotyledon (Be) is greatly 
raised, but is not otherwise changed in position. The left 
cotyledon (Lc) is likewise raised, but it is also twisted, so that 
its blade, instead of exactly facing the opposite one, now stands 
at nearly right angles to it This nocturnal twisting movement 
is effected not by means of the pulvinus, but by the twisting of 
the whole length of the petiole, as could be seen by the curved 
line of its upper concave surface. At the same time the true 
leaf (F) rises up, so as to stand vertically, or it even passes the 
vertical and is inclined a little inwards. It also twists a little, 
by which means the upper surface of its blade fronts, and 
almost comes into contact with, the upper surface of the twisted 



Chap. VI. SLEEP OF COTYLEDONS. 311 

left cotyledon. This seems to be the object gained by these 
singular movements. Altogether 20 seedlings were examined on 
successive nights, and in 19 of them it was the left cotyledon 
alone which became twisted, with the true leaf always so twisted 
that its upper surface approached closely and fronted that of the 
left cotyledon. In only one instance was the right cotyledon 
twisted, with the true leaf twisted towards it ; but this seedling 
was in an abnormal condition, as the left cotyledon did not rise 
up properly at night. This whole case is remarkable, as with 
the cotyledons of no other plant have we seen any nocturnal 
movement except vertically upwards or downwards. It is the 
more remarkable, because we shall meet with an analogous case 
in the leaves of the allied genus Melilotus, in which the ter- 
minal leaflet rotates at night so as to present one edge to the 
zenith and at the same time bends to one side, so that its upper 
surface comes into contact with that of one of the two now ver- 
tical lateral leaflets. 

Concluding Bemarks on the Nycfitropic . Movements of 
Cotyledons. — The sleep of cotyledons (though this is a 
subject which has been little attended to), seems to be 
a more common phenomenon than that of leaves. We 
observed the position of the cotyledons during the day 
and night in 153 genera, widely distributed through- 
out the dicotyledonous series, but otherwise selected 
almost by hazard ; and one or more species in 26 of 
these genera placed their cotyledons at night so as 
to stand vertically or almost vertically, having gene- 
rally moved through an angle of at least 60°. If we 
lay on one side the Leguminosee, the cotyledons of 
which are particularly liable to sleep, 140 genera 
remain ; and out of these, the cotyledons of at least one 
species in 19 genera slept. Now if we were to select 
by hazard 140 genera, excluding the Leguminosae, and 
observed their leaves at night, assuredly not nearly 
so many as 19 would be found to include sleeping 
species. We here refer exclusively to the plants 
observed by ourselves. 
21 



312 MODIFIED CIRCUMNTJTATION. Chap. VI 

In our entire list of seedlings, there are 30 genera, 
belonging to 16 Families, the cotyledons of which in 
some of the species rise or sink in the evening or 
early night, so as to stand at least 60° above or be- 
neath the horizon. In a large majority of the genera, 
namely, 24, the movement is a rising one ; so that 
the same direction prevails in these nyctitropic move- 
ments as in the lesser periodic ones described in the 
second chapter. The cotyledons move downwards 
during the early part of the night in only 6 of the 
genera ; and in one of them, Cannabis, the curving 
down of the tip is probably due to epinasty, as Kraus 
believes to be the case with the leaves. The down- 
ward movement to the amount of 90 J is very decided 
in Oxalis Valdiviana and sensitive*,, and in Geranium 
rotundifolium. It is a remarkable fact that with Anoda 
Wriglitii, one species of Gossypium and at least 3 
species of Ipomoea, the cotyledons whilst young and 
light sink at night very little or not at all ; although 
this movement becomes well pronounced as soon as 
they have grown large and heavy. Although the 
downward movement cannot be attributed to the 
weight of the cotyledons in the several cases which 
were investigated, namely, in those of the Anoda, 
Ipomoea purpurea and bona-nox, nor in that of I coc- 
cinea, yet bearing in mind that cotyledons are con- 
tinually circumnutating, a slight cause might at first 
have determined whether the great nocturnal move- 
ment should be upwards or downwards. We may 
therefore suspect that in some aboriginal member of 
the groups in question, the weight of the cotyledons 
first determined the downward direction. The fact of 
the cotyledons of these species not sinking down much 
whilst they are young and tender, seems opposed to 
the belief that the greater movement when they are 



Chap. VI. SLEEP OF COTYLEDONS. 313 

grown older, has been acquired for the sake of pro- 
tecting them from radiation at night ; but then we 
should remember that there are many plants, the 
leaves of which sleep, whilst the cotyledons do not ; 
and if in some cases the leaves are protected from cold 
at night whilst the cotyledons are not protected, so in 
other cases it may be of more importance to the species 
that the nearly full-grown cotyledons should be better 
protected than the young ones. 

In all the species of Oxalis observed by us, the coty- 
ledons are provided with pulvini ; but this organ has 
become more or less rudimentary in 0. corniculata, 
and the amount of upward movement of its cotyledons 
at night is very variable, but is never enough to be 
called sleep. We omitted to ascertain whether the 
cotyledons of Geranium rotundi folium possess pulvini. 
In the Leguminosse all the cotyledons which sleep, as 
far as we have seen, are provided with pulvini. But 
with Lotus Jacobmus, these are not fully developed 
during the first few days of the life of the seedling, 
and the cotyledons do not then rise much at night. 
With Trifolium striatum the blades of the cotyledons 
rise at night by the aid of their pulvini ; whilst the 
petiole of one cotyledon twists half-round at the same 
time, independently of its pulvinus. 

As a general rule, cotyledons which are provided 
with pulvini continue to rise or sink at night during 
a much longer period than those destitute of this organ. 
In this latter case the movement no doubt depends on 
alternately greater growth on the upper and lower side 
of the petiole, or of the blade, or of both, preceded 
probably by the increased turgesc'ence of the growing 
cells. Such movements generally last for a very 
short period — for instance, with Brassica and Githago 
for 4 or 5 nights, with Beta for 2 or 3, and with 



314 MODIFIED CIKCUMNUTATION. Chap VL 

Raplianus for only a single night. There are, however, 
some strong exceptions to this rule, as the cotyledons 
of Gossypium, Anoda and Ipomoaa do not possess pul- 
vini, yet continue to move and to grow for a long time. 
We thought at first that when the movement lasted for 
only 2 or 3 nights, it could hardly be of any service 
to the plant, and hardly deserved to be called sleep ; 
but as many quickly-growing leaves sleep for only a 
few nights, and as cotyledons are rapidly developed 
and soon complete their growth, this doubt now seems 
to us not well-founded, more especially as these move- 
ments are in many instances so strongly pronounced. 
We may here mention another point of similarity 
between sleeping leaves and cotyledons, namely, that 
some of the latter (for instance, those of Cassia and 
Githago) are easily affected by the absence of light ; 
and they then either close, or if closed do not open ; 
whereas others (as with the cotyledons of Oxalis) are 
very little affected by light. In the next chapter it 
will be shown that the nyctitropic movements both 
of cotyledons and leaves consist of a modified form of 
circumnutation. 

As in the Leguniinosse and Oxalidae, the leaves and 
the cotyledons of the same species generally sleep, the 
idea at first naturally occurred to us, that the sleep 
of the cotyledons was merely an early development of 
a habit proper to a more advanced stage of life. But 
no such explanation can be admitted, although there 
seems to be some connection, as might have been 
expected, between the two sets of cases. For the 
leaves of many plants sleep, whilst their cotyledons do 
not do so — of which fact Vesmodium gyrans offers a 
good instance, as likewise do three species of Nico- 
tiana observed by us ; also Sida rhombifolia, Abutilon 
Darwinii, and C/ienopodium album. On the other 



Cuai>. VI. SLEEP OF COTYLEDONS. 315 

hand, the cotyledons of some plants sleep and not the 
leaves, as with the species of Beta, Brassica, Geranium, 
Apium, Solanum, and Mirabilis, named in our list. 
Still more striking is the facfc that, in the same genus, 
the leaves of several or of all the species may sleep, 
but the cotyledons of only some of them, as occurs 
with Trifolium, Lotus, Gossypium, and partially with 
Oxalis. Again, when both the cotyledons and the 
leaves of the same plant sleep, their movements may 
be of a widely dissimilar nature : thus with Cassia the 
cotyledons rise vertically up at night, whilst their 
leaves sink down and twist round so as to turn their 
lower surfaces outwards. With seedlings of Oxalis 
Valdiviana, having 2 or 3 well-developed leaves, it 
was a curious spectacle to behold at night each leaflet 
folded inwards and hanging perpendicularly down- 
wards, whilst at the same time and on the same plant 
the cotyledons stood vertically upwards. 

These several facts, showing the independence of 
the nocturnal movements of the leaves and cotyledons 
on the same plant, and on plants belonging to the 
same genus, lead to the belief that the cotyledons have 
acquired their power of movement for some special 
purpose. Other facts lead to the same conclusion, 
such as the presence of pulvini, by the aid of which 
the nocturnal movement is continued during some 
weeks. In Oxalis the cotyledons of some species 
move vertically upwards, and of others vertically 
downwards at night ; but this great difference within 
the same natural genus is not so surprising as it 
may at first appear, seeing that the cotyledons of all 
the species are continually oscillating up and down 
during the day, so that a small cause might determine 
whether they should rise or sink at night. Again, the 
peculiar nocturnal movement of the left-hand coty- 



818 MODIFIED CIKCUMNUTATION. CiiAr VT, 

ledon of Trifolium, striatum, in combination with that 
of the first true leaf. Lastly, the wide distribution in 
the dicotyledonous series of plants with cotyledons 
which sleep. Reflecting on these several facts, our 
conclusion seems justified, that the nyctitropic move- 
ments of cotyledons, by which the blade is made to 
stand either vertically or almost vertically upwards 
or downwards at night, has been acquired, at least 
in most cases, for some special purpose ; nor can w& 
doubt that this purpose is the protection of the upper 
surface of the blade, and perhaps of the central bud 
or plumule, from radiation at night. 



Chai. vil modified ciecumnutation. 317 



CHAPTER VIL 

MoDiriED Circtmnuta^ion : Nyctitropic or Sleep MoVEBrENlS OF 
Leaves. 

Conditions necessary for these movements — List of Genera and Families, 
which include sleeping plants — Description of the movements in 
the several Genera — Oxalis : leaflets folded at night — Averrhoa : 
rapid movements of the leaflets — Porlieua: leaflets close when 
plant kept very dry — Tropseolum : leaves do not sleep unless well 
illuminated during day — Lupinus: various modes of sleeping — 
Melilotus : singular movements of terminal leaflet — Trifolium — 
Desmodium: rudimentary lateral leaflets, movements of, not de- 
veloped on young plants, state of their pulvini — Cassia : complex 
movements of the leaflets — Bauhinia: leaves folded at night — 
Mimosa pudica : compounded movements of leaves, effect of dark- 
ness — Mimosa albida, reduced leaflets of — Schrankia: downward 
movement of the pinnae — Marsilea : the only cryptogam known to 
sleep — Concluding remarks and summary — Nyctitropism consists 
of modified circum nutation, regulated by the alternations of light 
and darkness — Shape of first true leaves. 

We now come to the nyctitropic or sleep move- 
ments of leaves. It should be remembered that we 
confine this term to leaves which place their blades 
at night either in a vertical position or not more than 
30° from the vertical, — that is, at least 60° above or 
beneath the horizon. In some few cases this w 
effected by the rotation of the blade, the petiole not 
being either raised or lowered to any considerable 
extent. The limit of 30° from the vertical is obviously 
an arbitrary one, and has been selected for reasons 
previously assigned, namely, that when the blade 
approaches the perpendicular as nearly as this, ouly 
half a3 much of the surface is exposed at night to the 



318 MODIFIED C1RCUMNUTATIQN. Chap. Vli 

zenith and to free radiation as when the blade is 
horizontal. Nevertheless, in a few instances, leaves 
which seem to be prevented by their structure from 
moving to so great an extent as 60° above or beneath 
the horizon, have been included amongst sleeping 
plants. 

It should be premised that the nyctitropic move- 
ments of leaves are easily affected by the conditions 
to which the plants have been subjected. If the ground 
is kept too dry, the movements are much delayed 
or fail : according to Dassen,* even if the air is 
very dry the leaves of Impatiens and Malva are 
rendered motionless. Carl Kraus has also lately 
insisted f on the great influence which the quantity of 
water absorbed has on the periodic movements of 
leaves ; and he believes that this cause chiefly deter- 
mines the variable amount of sinking of the leaves of 
Polygonum convolvulus at night ; and if so, their move- 
ments are not in our sense strictly nyctitropic. Plants 
in order to sleep must have been exposed to a proper 
temperature : Erythrina crista-galli, out of doors and 
nailed against a wall, seemed in fairly good health, 
but the leaflets did not sleep, whilst those on another 
plant kept in a warm greenhouse were all vertically de- 
pendent at night. In a kitchen-garden the leaflets of 
Phaseolus vulgaris did not sleep during the early part 
of the summer. Ch. Koyer says,:j: referring I suppose 
to the native plants in France, that they do not sleep 
when the temperature is below 5° C. or 41° F. In 
the case of several sleeping plants, viz., species of 



* Dassen, ' Tijdschrift vor. Na- Bot.' (5th series), ix. 1868, p. 345. 

fcurlijke Gesch. en Physiologic,' f ' Beitriage zur Kentuiss der 

1837, vol. iv. p. 106. See also Bewegungen,' &c, in * Flora,' 

Ch. Royer on the importance of a 1879, pp. 42, 43, 67, &c. 

proper state of turgescence of the J ' Annal. des So. Nat. Bot.' 

cells, in 'Annal. det Sc. Nat. (5th Series), ix. 1868 p.H66. 



Chap. ML SLEEP OF LEAVES. 319 

Tropseolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia, 
and probably other genera, it is indispensable that 
the leaves should be well illuminated during the day 
in order that they may assume at night a vertical 
position ; and it was probably owing to this cause 
that seedlings of Chenopodium album and Siegesbeckia 
orientalis, raised by us during the middle of the winter, 
though kept at a proper temperature, did not sleep. 
Lastly, violent agitation by a strong wind, during a 
few minutes, of the leaves of Maranta arundinacea 
(which previously had not been disturbed in the hot- 
house), prevented their sleeping during the two next 
nights. 

We will now give our observations on sleeping 
plants, made in the manner described in the Intro- 
duction. The stem of the plant was always secured 
(when not stated to the contrary) close to the base of 
the leaf, the movements of which were being observed, 
so as to prevent the stem from circumnutating. As 
the tracings were made on a vertical glass in front of 
the plant, it was obviously impossible to trace its 
course as soon as the leaf became in the evening 
greatly inclined either upwards or downwards ; it 
must therefore be understood that the broken lines 
in the diagrams, which represent the evening and 
nocturnal courses, ought always to be prolonged to a 
much greater distance, either upwards or downwards, 
than appears in them. The conclusions which may be 
deduced from our observations will be given near the 
end of this chapter. 

In the following list all the genera which include 
sleeping plants are given, as far as known to us. The 
same arrangement is followed as in former cases, and 
the number of the Family is appended. This list 
possesses some interest, as it shows that the habit of 



320 



MODIFIED CIRCUMNUTATION. 



Chat VII. 



sleeping is common to some few plants throughout 
the whole vascular series. The greater number of the 
genera in the list have been observed by ourselves 
with more or less care ; but several are given on the 
authority of others (whose names are appended in the 
list), and about these we have nothing more to say. 
No doubt the list is very imperfect, and several genera 
might have been added from the ' Somnus Plantarum' 
by Linnaeus ; but we could not judge, iD some of his 
cases, whether the blades occupied at night a nearly 
vertical position. He refers to some plants as sleeping, 
for instance, Lathyrus odoratus and Viciafaba, in which 
we could observe no movement deserving to be called 
sleep, and as no one can doubt the accuracy of Linnseus, 
we are left in doubt. 



List of Genera, including species the leaves of which sleep 

Class I. DICOTYLEDONS. 
Sub-class I. Angiosperms. 
Family. 
Caryophylleae (26; 



Genus. 
Githago 

Stellaria (Batalin). 
Portulaca (Ch."l 

Royer). J 

Sida. 
Abutilon. 
Malva (Linnseusl 

and Pfeffer). J 
Hibiscus (Lin-1 

naeus). J 

Anoda. 
Gossypium. 
Ayenia (Linnaeus). 
Iriumfetta (Lin-l 

nams). / 

Linum (Batalin). 
Oxalis. 
Averrhoa. 
Porlieria. 
Guiacum. 
Impatieus (Lin 

na;us, Pfo Fer 

Batalin) 



Portulaceae (27). 
Malvaceae (36). 



Sterculacese (37). 

Tiliaceae (38).' 

Lineae (39). 
Oxalidse (41). 

Zygophylleae (45). 
Balsaminese (48). 



Sub-class I. Angiospeiuis — continued. 
Genus. \ Family. 

Tropaeolum. Tropaeoleae (49). 

Crotolaria (Thisel-\ Leguminosae (75) 



ton Dyer), 
Lupinus. 
Cytisus. 
Trigonella. 
Medicago. 
Melilotus. 
Trifolium. 
Securigera. 
Lotus. 
Psoralea. 
Amorpha 

chartre). 
Daslea. 
Indigofera. 
Tephrosia. 
Wistaria. 
Robinia. 
Sphaerophysa. 
Colutea. 
Astragalus. 
Glycyrvhiza. 
Coronilla. 
Hcdysarum. 



v 



(Du- 



Tribe II. 

Tr. 'ill. 

55 
Tr.'lV. 
Tr."\ r . 



Tr. VI 



Chap. Vll. 



SLEEP OF LEAVES. 



321 



List of Genera (continued). 



Cl^ss 1. DICOTYLEDONS (continued). 
Sub-class I. Angiospeums. 



Genus. 
Onobrychis. 

Smithia. 

Arachis. 

Desmodium. 

Urania. 

Vicia. 

Oentrosema. 

Amphicarpaea. 

Glycine. 

Erythrina. 

Apios. 

Phaseolus. 

Sophora. 

Oaesalpinia. 

Haematoxylon. 

Gleditsclna (Du- 

chartre). 
Poinciana. 
Cassia. 
Bauhinia. 
Tamarind up. 
Adenanthera. 
Prosopis. 
Neptunia. 
Mimosa. 
Schrankia. 
Acacia. 
Albizzia. 



Family. 
Leguminosse (75) 
Tr. VI. 



Tr. VII. 
Tr. VIII. 



Sub-class I. Angiospfims (continued), 



Tr. X. 

Tr. XII [. 



XIV. 
XV. 
XVI. 



Tr. XX. 



Tr. XXII. 
Tr. XXIII. 



Melaleuca(Bouche). Myrtaccae (94), 



Genus. 
iEnothera (Lin 

nams). 
PassiHora. 
Siegesbeckia. 

Ipomoea. 

Nicotiana. 
Mirabilis. 
Polygonum (Ba- 
talin). 

Amaranthus. 

Chenopodium. 
Pimelia (Bouche). 
Euphorbia. 
Phyllanthus(Pfef- 
fer). • 



""I 



Family. 
Onagrarieae (100). 

Passifloracese (105 s 
Composite (122). 
jConvolvulaceae 
\ (151). 
Solanea? (157). 
Nyctagineae (177). 

Polygoneac (179). 

(Amaranthaceaj 
\ (180). 
Chenopodiese (181) 
Thymeteae (188). 
Euphorbiaceae (202^ 



Sub-class II. Gymmosperms. 

Abies (Chatiu). 

Class 11. MONOCOTYLEDONS. 



Thalia. 


Cannaceae (21). 


Maranta. 


J? 


Colocasia. 


Aroideao (30). 


Strephium. 


Gramineae (55). 



Class III. ACOTYLEDOXS. 
Marsilea. Marsileaceae (4). 



Githago segetum (Caryophyllese). — The first leaves produced 
by young seedlings, rise up and close together at night. On a 
rather older seedlicg, two young leaves stood at noon at 55° 
above the horizon, and at night at 86°, so each had risen 31°. 
The angle, however, was less in some cases. Similar observations 
were occasionally made on young leaves (for the older ones moved 
very little) produced by nearly full-grown plants. Batalin 
says (' Flora/ Oct. 1st, 1873, p. 437) that the young leaves of 
Stellaria close up so completely at night that they form together 
great buds. 

Sida (Malvaceae). — The nyctitropic movements of the leaves 
in this genus are remarkable in some respects. Batalin informs 



322 



MODIFIED CIKCUMNUTATION. 



Chap. YII. 



us (see also 'Flora/ Oct. 1st, 1873, p. 437) that those oi 

S. napcea fall at night, but 



Fig. 126 



6"40 , a.m.20 t . 



\lO'fy.m.3S* 



6°d5'cf.m.30 t i\ 



8°S0'a.nr,.30 t ? t 



~9°l£'a,m.28 e ; 



tk 



.39^1 



Eirfa rhomhifolia : circumnu^ation and 



to what angle he cannot 
remember. The leaves of 
S. rhomhifolia and retusa, on 
the other hand, rise up 
vertically, and' are pressed 
against the stem. We have 
therefore here within the 
same genus, directly op- 
posite movements. Again, 
the leaves of S. rhomhifolia 
are furnished with a pul- 
vinus, formed of a mass of 
small cells destitute of chlo- 
rophyll, and with their 
longer axes perpendicular 
to the axis of the petiole. 
As measured along this 
latter line, these cells are 
only 1th of the length of 
those of the petiole; but 
instead of being abruptly 
separated from them (as is 
usual with the pulvinus in 
most plants), they graduate 
into the larger cells of the 
petiole. On the other hand, 
<S. napcea, according to Ba- 
talin, does not possess a 
pulvinus; and he informs 
us that a gradation may lie 
traced in the several species 
of the genus between these 



nyctitropic (or sleep) movements of two states of the petiole. 

!>ida rhomhifolia presents 
another peculiarity, of which 
we have seen no other in- 
stance with leaves that 
sleep: for those on very 
young plants, though they 
rise somewhat in the evening, do not go to sleep, as we observed 



leaf on a young plant, 9J inches 
high ; filament fixed to midrib of 
nearly full-grown leaf, 2| inches in 
length ; movement traced under a sky- 
light. Apex of leaf 5| inches from 
the vertical glass, so diagram not 
greatly enlarged. 



Chap. ML SLEEP OF LEAVES. 323 

on several occasions ; whilst those on rather older plants sleep 
in a conspicuous manner. For instance, a leaf ( - 85 of an inch 
in length) on a very young seedling 2 inches high, stood at noon 
9° above the horizon, and at 10 p.m. at 28°, so it had risen only 
19°; another leaf (1'4 inch in length) on a seedling of the 
same height, stood at the same two periods at 7° and 32°, and 
therefore had risen 25°. These leaves, which moved so little, 
had a fairly well-developed pulvinus. After an interval of some 
weeks, when the same seedlings were 2g and 3 inches in height, 
some of the young leaves stood up at night quite vertically, and 
others were highly inclined ; and so it was with bushes which 
were fully grown and were flowering. 

The movement of a leaf was traced from 9.15 a.m. on 
May 28th to 8.30 a.m. on the 30th. The temperature was too 
low (15° — 16° C), and the illumination hardly sufficient; con- 
sequently the leaves did not become quite so highly inclined at 
night, as they had done previously and as they did subse- 
quently in the hot-house ; but the movements did not apjoear 
otherwise disturbed. On the first day the leaf sank till 
5.15 p.m. ; it then rose rapidly and greatly till 10.5 p.m., and 
only a little higher during the rest of the night (Fig. 126). 
Early on the next day (29th) it fell in a slightly zigzag line 
rapidly until 9 a.m., by which time it had reached nearly the 
same place as on the previous morning. During the remainder 
of the day it fell slowly, and zigzagged laterally. . The evening 
rise began after 4 p.m. in the same manner as before, and on 
the second morning it again fell rapidly. The ascending and 
descending lines do not coincide, as may be seen in the diagram. 
On the 30th a new tracing was made (not here given) on a 
rather enlarged scale, as the apex of the leaf now stood 9 inches 
from the vertical glass. In order to observe more carefully the 
course pursued at the time when the diurnal fall changes into 
the nocturnal rise, dots were made every half-hour between 
4 p.m. and 10.30 p.m. This rendered the lateral zigzagging 
movement during the evening more conspicuous than in the 
diagram given, but it was of the same nature as there shown. 
The impression forced on our minds was that the leaf was 
expending superfluous movement, so that the great nocturnal 
rise might not occur at too early an hour. 

Abutilon Darwinii (Malvaceae). — The leaves on some very 
young plants stood almost horizontally during the day, and 
hung down vertically at night. Very fine jjlants kept in a 



324 MODIFIED CIRCUMNUTATION. Chap. VFT. 

large hall, lighted only from the roof, did not sleep at night, 
for in order to do so the leaves must be well illuminated during 
the day. The cotyledons do not sleep. Linnaeus says that the 
leaves of his Sida abutilon sink perpendicularly down at night, 
though the petioles rise. Prof. Pfeffer informs us that the 
leaves of a Malva, allied to M. sylvestris, rise greatly at night; 
and this genus, as well as that of Hibiscus, are included by 
Linnaeus in his list of sleeping plants. 

Anoda Wrightii (Malvaceae). — The leaves, produced by very 
young plants, when grown to" a moderate size, sink at night 
either almost vertically down or to an angle of about 45° beneath 
the horizon ; for there is a considerable degree of variability in 
the amount of sinking at night, which depends in part on the 
degree to which they have been illuminated during the day. 
But the leaves, whilst quite young, do not sink down at night, 
and this is a very unusual circumstance. The summit of the 
petiole, where it joins the blade, is developed into a pulvinus, 
and this is present in very young leaves which do not sleep ; 
though it is not so well defined as in older leaves. 

Gossypium (var. Nankin cotton, Malvaceae). — Some young 
leaves, between 1 and 2 inches in length, borne by two seedlings 
6 and 7 \ inches in height, stood horizontally, or were raised a 
little above the horizon at noon on July 8th and 9th ; but by 
10 p.m. they had sunk down to between 68° and 90° beneath 
the horizon. When the same plants had grown to double 
the above height, their leaves stood at night almost or quite 
vertically dependent. The leaves on some large plants of 
67. maritimum and Brazilense, which were kept in a very badly 
lighted hot-house, only occasionally sank much downwards 
at night, and hardly enough to be called sleep. 

Oxalis (Oxalidae).— In most of the species in this large genus 
the three leaflets sink vertically down at night; but as their 
sub-petioles are short the blades could not assume this position 
from the want of space, unless they were in some manner ren- 
dered narrower; and this is effected by their becoming more 
or less folded (Fig. 127). The angle formed by the two halves 
of the same leaflet was found to vary in different individuals of 
several species between 92° and 150°; in three of the best 
folded leaflets of 0. fragrans it was 76°, 74°, and 54°. The 
angle is often different in the three leaflets of the same leaf. 
As the leaflets sink down at night and become folded, their 
lower surfaces are brought near together (see B), or even into 



CllAV. VII. 



SLEEP OF LEAVES. 



325 



close contact; and from this circumstance it might be thought 
that the object of the folding was the protection of their lower 
surfaces. If this had been the case, it would have formed 
a strongly marked exception to the rule, that when there is any 
difference in the degree of protection from radiation of the two 
surfaces of the leaves, it is always the upper surface which is 
the best protected. But that the folding of the leaflets, and 
consequent mutual approximation of their lower surfaces, 
serves merely to allow them to sink down vertically, may be 

Fig. 127. 





A. B. 

Oxalis acetoseUa: A, leaf seen from vertically above; B, diagram of leaf 
asliep, also seen from vertically above. 

inferred from the fact that when the leaflets do not radiate 
from the summit of a common petiole, or, again, when there is 
plenty of room, from the sub-petioles not being very short, the 
leaflets sink down without becoming folded. This occurs with 
the leaflets of 0. sensitiva, Plumierii, and bupleurifolia. 

There is no use in giving a long list of the many species 
which sleep in the above described manner. This holds good 
with species having rather fleshy leaves, like those of 0. carnosa, 
or large leaves like those of 0. Or tf genii, or four leaflets like 
those of 0. variabilis. There are, however, some species which 
show no signs of sleep, viz., 0. peniaphylla, enneaphylla, hirta, 
and rubella. We will now describe the nature of the movements 
in some of the species. 

Oxalis acetoseUa. — The movement of a leaflet, together with 
that of the main petiole, are shown in the following dia- 
gram (Fig. 128), traced between 11 a.m. on October 4th and 
7.45 a.m. on the 5th. After 5.30 p.m. on the 4th the leaflet sank 
rapidly, and at 7 p.m. depended vertically. Fw some time 
before it assumed this latter position, its movements could, of 
course, no longer be traced on the vertical glass, and the 
broken line in the diagram ought to be extended much further 



326 



MODIFIED CIRCUMNUTATiON 
Fig. 128. 

JSbun A l h 



Chap. VTI, 




\Q a A5 y a.m.5 [ 
Oxalis acetosella: circumnutation and 
nyctitropic movements of a nearly- 
full-grown leaf, with filament at- 
tached to the midrib of one of the 
leaflets; traced on vertical glass dur- 
ing 20 h. 45 m. 



down in this and all other 
cases. By 6.45 a.m. on the 
following morning it had 
risen considerably, and con- 
tinued to rise for the next 
hour; but, judging from 
other observations, it would 
soon have begun to fall again. 
Between 11 a.m. and 5.30 p.m. 
the leaflet moved at least four 
times up and four times 
down before the great noc- 
turnal fall commenced; it 
reached its highest point at 
noon. Similar observations 
were made on two other 
leaflets, with nearly the same 
results. Sachs and Pfeffer 
have also described briefly* 
the autonomous movements 
of the leaves of this plant. 

On another occasion the 
petiole of a leaf was secured 
to a little stick close beneath 
the leaflets, and a filament 
tipped with a bead of sealing- 
wax was affixed to the mid- 
rib of one of them, and a 
mark was placed close behind. 
At 7 p.m., when the leaflets 
were asleep, the filament de- 
pended vertically down, and 
the movements of the bead 
were then traced till 10.40 
p.m., as shown in the fol- 
lowing diagram (Fig. 129). 
We here see that the leaflet 
moved a little from side to 
side, as well as a little up 
and down, whilst asleep. 



* Sachs in 'Flora,' 1863, p. 470, &c. ; Pfeffer, 'Die Period. Bewe- 
Kungen,' &c, 1875, p. 53. 




Chap. VII. SLEEP OF LEAVES. 327 

Oxalis Vdldiviana. — The leaves resemble those of the last 
species, and the movements of two leaflets (the main petioles of 
both having been secured) were 
traced during two days ; but the Fig- 129. 

tracings are not given, as they 
resembled that of 0. acetosella, with 
the exception that the up and 
down oscillations were not so fre- 
quent during the day, and there Oxalis acetosella: circumnuta- 
was more lateral movement, so that * ion °/ leaflet wh + en 1 asle «P 5 

, ... -. .-, n traced on vertical glass 

broader ellipses were described. during 3 h. 40 m. 
The leaves awoke early in the morn- 
ing, for by 6.45 a.m. on June 12th and 13th they had not only 
risen to their full height, but had already begun to fall, that is, 
they were circumnutating. We have seen in the last chapter 
that the cotyledons, instead of sinking, rise up vertically at 
night. 

Oxalis Ortegesii. — The large leaves of this plant sleep like 
those of the previous species. The main petioles are long, and 
that of a young leaf rose 20° between noon and 10 p.m., whilst 
the petiole of an older leaf rose only 13°. Owing to this rising 
of the petioles, and the vertical sinking of the large leaflets, 
the leaves become crowded together at night, and the whole 
plant then exposes a much smaller surface to radiation than 
during the day. 

Oxalis Plumierii. — In this species the three leaflets do not 
surround the summit of the petiole, but the terminal leaflet 
projects in the line of the petiole, with a lateral leaflet on each 
side. They all sleep by bending vertically downwards, but 
do not become at all folded. The petiole is rather long, and, 
one having been secured to a stick, the movement of tfre terminal 
leaflet was traced during 45 h. on a vertical glass. It moved 
in a very simple manner, sinking rapidly after 5 p.m., and 
rising rapidly early next morning. During the middle of the day 
it moved slowly and a little laterally. Consequently the ascend- 
ing and descending lines did not coincide, and a single great 
ellipse was formed each day. There was no other evidence of 
circumnutation, and this fact is of interest, as we shall here- 
after see. 

Oxalis sensitiva. — The leaflets, as in the last species, bend 
vertically down at night, without becoming folded. The much 
elongated main petiole rises considerably in the evening, but in 
22 



32S 



MODIFIED CIKCUMNTJTATION. 



Chap. VII 



Fig. 130. 



some very young plants the rise did not commence until late 
at night. We have seen that the cotyledons, instead of sink- 
ing like the leaflets, rise up vertically at night. 

Oxalis bupleurifolia. — This species 
is rendered remarkable by the petioles 
being foliaceous, like the phyllcdes 
of .many Acacias. The leaflets are 
small, of a paler green and more 
tender consistence than the folia- 
ceous petioles. The leaflet which was 
observed was * 55 inch in length, and 
was borne by a petiole 2 inches long 
and '3 inches broad. It may be 
suspected that the leaflets are on the 
road to abortion or obliteration, as 
has actually occurred with those of 
another Brazilian species, 0. msci- 
formis. Nevertheless, in the present 
species the nyctitropic movements 
are perfectly performed. The folia- 
ceous petiole was first observed 
during 48 h., and found to be in 
continued circumnutation, as shown 
in the accompanying figure (Fig. 
130). It rose during the day and 
early part of the night, and fell 
during the remainder of the night 
and early morning; but the move- 
ment was not sufficient to be called 
sleep. The ascending and descend- 
ing lines did not coincide, so that an 
ellipse was formed each day. There 
was but little zigzagging; if the 
filament had been fixed longitudi- 
nally, we should probably have seen 
that there was more lateral move- 
ment than appears in the diagram. 
A terminal leaflet on another leaf was next observed (the 
petiole being secured), and its movements are shown in 
Fig. 131. During the day the leaflets are extended horizon- 
tally, and at night depend vertically ; and as the petiole rises 
daring the day the leaflets have to bend down in the evening 



Oxalis bupleurifolia: circum- 
autation of foliaceous pe- 
tiole, filament fixed ob- 
liquely across end of petiole ; 
movements traced on ver- 
tical glass from 9. A.M. June 
26th to 8.50 A.M. 28th. 
Apex of leaflet 4£ inches 
from the glass, so movement 
not much magnified. Plant 
9 inches high, illuminated 
fmm above. Temp. 23|°- 
24£° C. 



Chap. VII 



SLEEP OE LEAVES. 



32§ 



more than 90°, so as to assume at night their vertical position. 
On the first day the leaflet simply moved up and down ; on the 

Fig. 131. 




T3 &0 



Sag 



.e -2 to 

S3 « 

'a. £ rt 

s 1 S 

•2 -*> M 

1 



second day it plainly circunmutated between 8 a.m. and 4.30 p.m. 
after which hour the great evening fall commenced. 



330 



MODIFIED CIECUMNUTATION. 



Chap. VII 



Averrhoa bilimhi (Oxalidse). — It has long been known,* firstly, 
that the leaflets in this genus sleep ; secondly, that they move 
spontaneously during the day ; and thirdly, that they are sensi- 
tive to a touch; but in none cf these respects do they diffei 
essentially from the species of Oxalis. They differ, however, as 
Mr. E. I. Lynch f has lately shown, in their spontaneous move- 
ments being strongly marked. In the case of A. Ulirribi, it is a 
wonderful spectacle to behold on a warm sunny day the leaflets 
one after the other sinking rapidly downw r ards, and again 
ascending slowly. Their movements rival those of Desmodium 
gyrans. At night the leaflets hang vertically down ; and now 

Fig. 132. 




Averrho2 bitimbi : loaf asleep ; drawing reduced. 

they are motiouless, but this may be due to the opposite ones 
being pressed together (Fig. 132). The main petiole is in con- 
stant movement during the day, but no careful observations were 
made on it. The following diagrams are graphic representa- 
tions of the variations in the angle, which a given leaflet makes 
with the vertical. The observations were made as follows. 
The plant growing in a pot was kept in a high temperaturo, 
the petiole of the leaf to be observed pointing straight at 
the observer, being separated from him by a vertical pane of 
glass. The petiole was secured so that the basal joint, or pul- 
vinus, of one of the lateral leaflets was at the centre of a gradu- 
ated arc placed close behind the leaflet. A fine glass filament 
was fixed to the leaf, so as to project like a continuation of the 



* Dr. Bruce, 'Philosophical Trans.,' 1785, p. 356. 
'Journal Linn. Sue.,' vol. xvi. 1877, p. 231. 



Chap. VII. 



SLEEP OF LEAVES. 



331 



midrib. This filament acted as an index ; and as the leaf rose 
a v nd fell, rotating about its basal joint, its angular movement 

Fie. 133. 




Averrkoa bilimbi: angular movements of a leaflet during its evening 
descent, when going to sleep. Temp. 78°-81° F. 



could be recorded by reading off at short intervals of time the 
position of the glass filament on the graduated arc. In order 



332 MODIFIED CIRCUMNUTATION. Chap. VII 

to avoid errors of parallax, all readings were made by looking 
through a small ring painted on the vertical glass, in a lino 
with the joint of the leaflet and the centre of the graduated arc. 
In the following diagrams the ordinates represent the angles 
which the leaflet made with the vertical at successive instants.* 
It follows that a fall in the curve represents an actual dropping 
of the leaf, and that the zero line represents a vertically do- 
pendent position. Fig. 133 represents the nature of the move- 
ments which occur in the evening, as soon as the leaflets begin 
to assume their nocturnal position. At 4.55 p m. the leaflet 
formed an angle of 85° with the vertical, or was only 5° below 
the horizontal ; but in order that the diagram might get into 
our page, the leaflet is represented falling from 75° instead 
of 85°. Shortly after 6 p m. it hung vertically down, and had 
attained its nocturnal position. Between 6.10 and 6.35 p.m. it 
performed a number of minute oscillations of about 2° each, 
occupying periods of 4 or 5 m. The complete state of rest of 
the leaflet which ultimately followed is not shown in the dia- 
gram. It is manifest that each oscillation consists of a gradual 
rise, followed by a sudden fall. Each time the leaflet fell, it 
approached nearer to the nocturnal position than it did on the 
previous fall. The amplitude of the oscillations diminished, 
while the periods of oscillation became shorter. 

In bright sunshine the leaflets assume a highly inclined de- 
pendent position. A leaflet in diffused light was observed rising 
for 25 m. A blind was then pulled up so that the plant was 
brightly illuminated (BR in Fig. 134), and within a minute it 
began to fall, and ultimately fell 47°, as shown in the diagram. 
This descent was performed by six descending steps, precisely 
similar to those by which the nocturnal fall is effected. The 
pkmt was then again shaded (SH), and a long slow rise occurred 
until another series of falls commenced at BR', when the sun 
was again admitted. In this experiment cool air was allowed 
to enter by the windows being opened at the same time that 
the blinds were pulled up, so that in spite of the sun shining 
on the plant the temperature was not raised. 

The effect of an increase of temperature in diffused light is 



* In all the diagrams 1 mm. in ment. In Figs. 133 and 134 the 

the horizontal direction represents temperature is represented (along 

one minute of time. Each mm. the ordinates) in the scale of 1 

in the vertical direction repre- mm. to each , l°O. In Fig. 

sents one degree of angular move- 135 each mm. equals 0'2° F. 



Chap. VII. SLEEP OF LEAVES. 333 

shown in Fig. 135. The temperature began to rise at 11.35 
a.m. (in consequence of the fire being lighted), but by 12.42 a 
marked fall had occurred. It may be seen in the diagram that 
when the temperature was highest there were rapid oscillations 




Averrhoa hiUmbi : angular movements of leaflet during a change from 
bright illumination to shade ; temperature (broken line) remaining 
nearly the same. 

of small amplitude, the mean position of the leaflet being at the 
time nearer the vertical. When the temperature began to fall, 
the oscillations became slower and larger, and the mean position 
of the leaf again approached the horizontal. The rate of oscil- 
lation was sometimes, quicker than is represented in the above 
diagram. Thus, when the temperature was between 31° rnd 



334 



MODIFIED CIRCUMNUTATION. 

Fig. 135. 



Chap. VII. 




Chap. VII. 



SLEEP OF LEAVES. 



33i 



Fig. 136. 



32° C, 14 oscillations of a few degrees occurred in 19 m. On 
the other hand, an oscillation may be much slower ; thus a leaflcJ 
was observed (temperature 25° C.) to 
rise during 40 m. before it fell and 
completed its oscillation. 

Porlieria hygrometrica (Zygophylleae) 
— The leaves of this plant (Chilian 
form) are from 1 to lj inches in length, 
and bear as many as 16 or 17 small 
leaflets on each side, which do not 
stand opposite one another. They are 
articulated to the petiole, and the 
petiole to the branch by a pulvinus. 
We must premise that apparently two 
forms are confounded under the same 
name : the leaves on a bush from Chili, 
which was sent to us from Kew, bore 
many leaflets, whilst those on plants 
in the Botanic Garden at "Wiirzburg 
bore only 8 or 9 pairs ; and the whole 
character of the bushes appeared some- 
what different. We shall also see that 
they differ in a remarkable physio- 
logical peculiarity. On the Chilian 
plant the petioles of the younger leaves 
on upright branches, stood horizontally 
during the day, and at night sank 
down vertically so as to depend parallel 
and close to the branch beneath. The 
petioles of rather older leaves did not 
become at night vertically depressed, 
but only highly inclined. In one 
instance we found a branch which had Polierla hygrometrica : oir- 
grown perpendicularly downwards, cumnutation and nycti- 
1 ,, ,. , ., t . ,, tropic movements ot pe- 

and the petioles on it moved in the same t iole of leaf, traced from 
direction relatively to the branch as 9.35 a.m. July 7th to 
just stated, and therefore moved up- 
wards. On horizontal branches the 
younger petioles likewise move at night 
in the same direction as before, that is, 
towards the branch, and are consequently then extended hori- 
zontally; but it is remarkable that the older petioles on the 




about midnight on the 
8th. Apex of leaf 7 J 
inches from the vertical 
glass. Temp.l9£°-20j°C. 



336 MODIFIED CIRCUMNUTATION. Chap. VII. 

same branch, though moving a little in the same direction, also 
bend downwards ; they thus occupy a somewhat different posi- 
tion, relatively to the centre of the earth and to the branch, from 
that of the petioles on the upright branches. With respect to 
the leaflets, they move at night towards the apex of the petiole 
until their midribs stand nearly parallel to it; and they then 
lie neatly imbricated one over the other. Thus half of the upper 
surface of each leaflet is in close contact with half of the lower 
surface of the one next in advance ; and all the leaflets, except- 
ing the basal ones, have the whole of their upper surfaces and 
half of their lower surfaces well protected. Those on the oppo- 
site sides of the same petiole do not come into close contact 
at night, as occurs with the leaflets of so many Leguminosas, 
but are separated by an open furrow; nor could they exactly 
coincide, as they stand alternately with respect to one another. 

The circumnutation of the petiole of a leaf f of an inch in 
length, on an upright branch, was observed during 36 h., 
and is shown in the preceding diagram (Fig. 136). On the 
first morning, the leaf fell a little and then rose until 1 p.m , 
and this was probably due to its being now illuminated through 
a skylight from above; it then circumnutated on a very small 
scale round the same spot until about 4 p.m., when the great 
evening fall commenced. During the latter part of the night or 
very early on the next morning the leaf rose again. On the 
second day it fell during the morning till 1 p.m., and this no 
doubt is its normal habit. From 1 to 4 p.m. it rose in a zigzag 
line, and soon afterwards the great evening fall commenced. It 
thus completed a double oscillation during the 2-4 h. 

The specific name given to this plant by Ptuiz and Pavon, indi- 
cates that in its native arid home it is affected in some manner 
by the dryness or dampness of the atmosphere.* In the Botanic 
Garden at Wtirzburg, there was a plant in a pot out of doors 
which was daily watered, and another in the open ground which 
was never watered. After some hot and dry weather there was 
a great difference in the state of the leaflets on these two plants; 
those on the un watered plant in the open ground remaining half, 



* ' Systema Vcg. Florse Peru- about its power of foretell ng 

vianse et Cliilensis,' torn. i. p. 95, changes in the weather; and it 

1708. We cannot understand the appe.irs as if the brightness of the 

account given by the authors of sky largely determined the opeu 

the behuviuu- of this plant in its ing and closing of the leaflets, 
native home There is much 



Ch^. VII. SLEEP OF LEAVES. 337 

or even quite, closed during the day. But twigs cut from tins 
bush, with their ends standing in water, or wholly immersed in 
it, or kept in damp air under a bell-glass, opened their leaves 
though exposed to a blazing sun; whilst those on the plant 
in the ground remained closed. The leaves on this same plant, 
after some heavy rain, remained open for two days; they then 
became half closed duriDg two days, and after an additional 
day were quite closed. This plant was now copiously wateied, 
and on the following morning the leaflets were fully ex- 
panded. The other plant growing in a pot, after having been 
exposed to heavy rain, was placed before a window in the Labo- 
ratory, with its leaflets open, and they remained so during the 
daytime for 48 h. ; but after an additional day were half closed. 
The plant was then watered, and the leaflets on the two following 
days remained open. On the third day they were again half 
closed, but on being again watered remained open during the 
two next days. From these several facts we may conclude that 
the plant soon feels the want of water ; and that as soon as this 
occurs, it partially or quite closes its leaflets, which in their 
then imbricated condition expose a small surface to evaporation. 
It is therefore probable that this sleep-like movement, which 
occurs only when the ground is dry, is an adaptation against 
the loss of moisture. 

A bush about 4 feet in height, a native of Chili, which was 
thickly covered with leaves, behaved very differently, for during 
the day it never closed its leaflets. On July 6th the earth ir. 
the small pot in which it grew appeared extremely dry, and 
it was given a very little water. After '21 and 22 days (on 
the 27th and 28th), during the whole of which time the plant 
did not receive a drop of water, the leaves began to droop, but 
they showed no signs of closing during the day. It appeared 
almost incredible that any plant, except a fleshy one, could 
have kept alive in soil so dry, which resembled the dust on 
a road. On the 29th, when the bush was shaken, some leaves 
fell off, and the remaining ones were unable to sleep at night. 
It was therefore moderately watered, as well as syringed, late in 
the evening. On the next morning (30th) the bush looked as fresh 
as ever, and at night the leaves went to sleep. It may be added 
that a small branch while growing on the bush was enclosed, 
by means of a curtain of bladder, during 13 days in a large 
bottle half full of quicklime, so that the air within must have been 
intensely dry ; yet the leaves on this branch did not suffer in the 



338 MODIFIED CIKCUMNUTATION. Chap. V1J 

least, and did not close at all during the hottest days. Anothei 
trial was made with the same bush on August 2nd and 6th (the soil 
appearing at this latter date extremely dry), for it was exposed 
out of doors during the whole day to the wind, but the leaflets 
showed no signs of closing. The Chilian form therefore differs 
widely from the one at Wiirzburg, in not closing its leaflets 
when suffering from the want of water ; and it can live for a 
surprisingly long time without water. 

Tropceolum majus (?) (cultivated var.) (Tropasoleae). — Several 
plants in pots stood in the greenhouse, and the blades of 
the leaves which faced the front-lights were during the day 
highly inclined and at night vertical ; whilst the leaves on 
the back of the pots, though of course illuminated through 
the roof, did not become vertical at night. We thought, at first, 
that this dfference in their positions was in some manner 
due to heliotropism, for the leaves are highly heliotropic. The 
true explanation, however, is that unless they are well illu- 
minated during at least a part of the .day they do not sleep at 
night; and a little difference in the degree of illumination deter- 
mines whether or not they shall become vertical at night. We 
have observed no other so well-marked a case as this, of the 
influence of previous illumination on nyctitropic movements. 
The leaves present also another peculiarity in their habit of 
rising or awaking in the morning, being more strongly fixed or 
iuherited than that of sinking or sleeping at night. The move- 
ments are caused by the bending of an upper part of the petiole, 
between £ and 1 inch in length ; but the part close to the blade, 
for about £ of an inch in length, does not bend and always 
remains at right angles to the blade. The bending portion does 
not present any external or internal difference in structure 
from the rest of the petiole. We will now give the experiments 
on which the above conclusions are founded. 

A large pot with several plants was brought on the morning 
of Sept. 3rd out of the greenhouse and placed before a north-east 
window, in the same position as before with respect to the light, 
as far as that was possible. On the front of the plants, 24 leaves 
were marked with thread, some of which had their blades hori- 
zontal, but the greater number were inclined at about 45°, 
beneath the horizon; at night all these, without exception, 
became vertical. Early on the following morning (4th) they 
reassumed their former positions, and at night again became 
vertical. On the 5th the shutters were opened at 6.15 a.m., and 



Chap. VII. SLEEP OF LEAVES. 339 

by 8.18 a.m., after the leaves had been illuminated for 2 h. 3 m., 
and had acquired their diurnal position, they were placed in a 
dark cupboard. They were looked at twice during the day and 
thrice in the evening, the last time at 10 30 p.m., and not one had 
become vertical. At 8 a.m. on the following morning (6th) they 
still retained the same diurnal position, and were now replaced 
before the north-east window. At night all the leaves which 
had faced the light had their petioles curved and their blades 
vertical ; whereas none of the leaves on the back of the plants, 
although they had been moderately illuminated by the diffused 
light of the room, were vertical. They were now at night placed 
in the same dark cupboard ; at 9 a.m. on the next morning (7th) 
all those which had been asleep had reassumed their diurnal 
position. The pot was then placed for 3 h. in the sunshine, so 
as to stimulate the plants ; at noon they were placed before the 
same north-east window, and at night the leaves slept in the 
usual manner and awoke on the following morning. At noon on 
this day (8th) the plants, after having been left before the north- 
east window for 5 h. 45 m. and thus illuminated (though not 
brightly, as the sky was cloudy during the whole time), were 
replaced in the dark cupboard, and at 3 p. m. the position of the 
leaves was very little, if at all, altered, so that they are not 
quickly affected by darkness ; but by 10.15 p.m. all the leaves 
which had faced the north-east sky during the 5h. 45 m. of 
illumination stood vertical, whereas those on the back of the 
plant retained their diurnal position. On the following morning 
(9th) the leaves awoke as on the two former occasions in the dark, 
and they were kept in the dark during the whole day ; at night 
a very few of them became vertical, and this was the one in- 
stance in which we observed any inherited tendency or habit in 
this plant to sleep at the proper time. That it was real sleep 
was shown by these same leaves reassuming their diurnal posi- 
tion on the following morning (10th) whilst still kept in the 
dark. 

The pot was then (9.45 a.m. 10th) replaced, after having been 
kept for 36 h. in darkness, before the north-east window ; and at 
night the blades of all the leaves (excepting a few on the back of 
the plants) became conspicuously vertical. 

At 6.45 a.m. (11th) after the plants had been illuminated on the 
same side as before during only 25 m., the pot was turned round, 
so that the leaves which had faced the light now faced the 
interior of the room, and not one. of these went to sleep at night; 



BIO MODIFIED CIKCUMXUTATIOK. Chap. VII. 

whilst some, but not many, of those which had formerly stood 
facing the back of the room and which had never before been 
well illuminated or gone to sleep, now assumed a vertical posi- 
tion at night. On the next day (12th) the plant was turned 
round into its original position, so that the saTne leaves faced 
the light as formerly, and these now went to sleep in the usual 
manner. We will only add that with some young seedlings 
kept in the greenhouse, the blades of the first pair of true leaves 
(the cotyledons being hypogean) stood during the day almost 
horizontally and at night almost vertically. 

A few observations were subsequently made on the circum- 
nutation of three leaves, whilst facing a north-east window ; but 
the tracings are not given, as the leaves moved somewhat 
towards the light. It was, however, manifest that they rose 
and fell more than once during the daytime, the ascending and 
descending lines being in parts extremely zigzag. The nocturnal 
fall commenced about 7 p.m., and the leaves had risen consider- 
ably by 6.45 a.m. on the following morning. 

Leguminosce. — This Family includes many more genera with 
sleeping species than all the other families put together. The 
number of the tribes to which each genus belongs, according to 
Bentham and Hooker's arrangement, has been added. 

Crotolaria (sp. ?) (Tribe 2). — This plant is monophyllous, and 
we are informed by Mr. T. Thiselton Dyer that the leaves rise 
up vertically at night and press against the stem. 

Lupinus (Tribe 2). — The palmate or digitate leaves of the 
species in this large genus sleep in three different manners. 
One of the simplest, is that all the leaflets become steeply in- 
clined downwards at night, having been during the day ex- 
tended horizontally. This is shown in the accompanying 
figures (Fig. 137), of a leaf of L. pilosus, as seen during the 
day from vertically above, and of another leaf asleep with the 
leaflets inclined downwards. As in this position they are 
crowded together, and as they do not become folded like those 
in the genus Oxalis, they cannot occupy a vertically dependent 
position ; but they are often inclined at an angle of 50° beneath 
the horizon. In this species, whilst the leaflets are sinking, 
the petioles rise up, in two instances when the angles were 
measured to the extent of 23°. The leaflets of L. sub-camosus and 
arboreus, which were horizontal during the day, sank down at 
night in nearly the same manner ; the former to an angle of 38°, 
and the latter of 36°, beneath the horizon; but their peticles 



Chap. VII. SLEEP OF LEAVES. 341 

did not move in any plainly perceptible degree. It is, however, 
quite possible, as we sliall presently see, that if a large number 
of plants of the three foregoing and of the following species 

Fig. 137. 





A. 

Lupinus pilosus : A, leaf seen from vertically above in daytime; B, leaf 
asleep, seen laterally at night. 

were to be observed at all seasons, some of the leaves would be 
found to sleep in a different manner. 

In the two following species the leaflets, instead of moving 
downwards, rise at night. With L. Eartwegii some stood at 
noon at a mean angle of 36° above the horizon, and at night 
at 51°, thus forming together a hollow cone with moderately 
steep sides. The petiole of one leaf rose 14° and of a second 
11° at night. "With L. luteus a leaflet rose from 47° at noon to 
65° above the horizon at night, and another on a distinct leaf 
rose from 45° to 69°. The petioles, however, sink at night to 
a small extent, viz., in three instances by 2°, 6°, and 9° 30'. 
Owing to this movement of the petioles, the outer and longer 
leaflets have to bend up a little more than the shorter and inner 
ones, in order that all should stand symmetrically at night. 
We shall presently see that some leaves on the same individual 
plants of L. luteus sleep in a very different manner. 

We now come to a remarkable position of the leaves 
when asleep, which is common to several species of Lupines. 
On the same leaf the shorter leaflets, which generally face the 
centre of the plant, sink at night, whilst the longer ones 
on the opposite side rise; the intermediate and lateral ones 
merely twisting on their own axes. But there is some variability 
with respect to which leaflets rise or fall. As might have been 
expected from such diverse and complicated movements, the 



342 



MODIFIED C1KCUMNUTATION. 



Chap. VI t . 



base of each leaflet is developed (at least in the case of L. luteus) 
into a pulvimis. The result is that all the leaflets on the 
same leaf stand at night more or less highly inclined, or even 
quite vertically, forming in this latter case a vertical star. This 
occurs with the leaves of a species purchased under the name of 



Fig. 138 




Lupinns pubescens: A, leaf viewed laterally during the day; B, same leaf 
at night ; C, another leaf with the leaflet forming a vertical star at 
night. Figures reduced. 

//. pubescens ; and in the accompanying figures we see at A (Fig. 
138) the leaves in their diurnal position; and at B the same 
plant at night with the two npper leaves having their leaflets 
almost vertical. At C another leaf, viewed laterally, is shown 
with the leaflets quite vertical. It is chiefly or exclusively the 
youngest leaves which form at night vertical stars. But there 



Chap VII. SLEEP OF LEAVES. 343 

is much variability in the position of the leaves at night on the 
same plant ; some remaining with their leaflets almost horizontal, 
others forming more or less highly inclined or vertical stars, and 
some with all their leaflets sloping downwards, as in our first 
class of cases. It is also a remarkable fact, that although all the 
plants produced from the same lot of seeds were identical m 
appearance, yet some individuals at night had the leaflets of all 
their leaves arranged so as to form more or less highly inclined 
stars ; others had them all sloping downwards and never forming 
a star ; and others, again, retained them either in a horizontal 
position or raised them a little. 

We have as yet referred only to the different positions of tne 
leaflets of L. pubescens at night ; but the petioles likewise differ 
in their movements. That of a young leaf which formed a 
highly inclined star at night, stood at noon at 42° above the 
horizon, and during the night at 72°, so had risen 30°. The 
petiole of another leaf, the leaflets of which occupied a similar 
position at night, rose only 6°. On the other hand, the petiole 
of a leaf with all its leaflets sloping down at night, fell at this 
time 4°. The petioles of two rather older leaves were subse- 
quently observed ; both of which stood during the day at exactly 
the same angle, viz., 50° above the horizon, and one of these rose 
7°-8°, and the other fell 3°— 4° at night. 

We meet with cases like that of L. pubescens with some other 
species. On a single plant of L. mutabilis some leaves, which 
stood horizontally during the day, formed highly inclined stars 
at night, and the petiole of one rose 7°. Other leaves which 
likewise stood horizontally during the day, had at night all theii 
leaflets sloping downwards at 46° beneath the horizon, but 
their petioles had hardly moved. Again, L. hit us offered a still 
more remarkable case, for on two leaves, the leaflets which stood 
at noon at about 45° above the horizon, rose at night to 65° and 
69°, so that they formed a hollow cone with steep sides. Four 
leaves on the same plant, which had their leaflets horizontal at 
noon, formed vertical stars at night; and three other leaves 
equally horizontal at noon, had all their leaflets sloping down- 
wards at night. So that the leaves on this one plant assumed 
at night three different positions. Though we cannot account 
for this fact, we can see that such a stock might readily give 
birth to species having widely different nyctitropic habits. 

Little more need be said about the sleep of the species of Lu- 
pinus; several, namely, L. polyphyllus, nanus, Menziesii, speciosus t 

23 



344 



MODIFIED CIRoUMtfUTATlON. 



Chap. VII 



and albifrons, though observed out of doors and in the green- 
house, did not change the position of their leaves sufficiently at 
night to be said to sleep. • From observations made on two 
sleeping species, it appears that, as with Tropceolum majus, the 
leaves must be well illuminated during the day in order to p!eep 
at night. For several plants, kept all day in a sitting-room 
with north-east windows, did not sleep at night ; but when the 
pots were placed on the following day out of doors, and were 
brought in at night, they slept in the usual manner. The trial 
was repeated on the following day and night with the same 
result. 

Some observations were made on the circumnutation of the 
leaves of L. luteus and arb>reus. It will suffice to say that the 
leaflets of the latter exhibited a double oscillation in the course 
of 24 h. ; for they fell from the early morning until 10 15 a.m., 
then rose and zigzagged greatly till 4 p.m., after which hour the 
great nocturnal fall commenced. By 8 a.m. on the following 
morning the leaflets had risen to their proper height. We have 
seen in the fourth chapter, that the leaves of Lupinus spcciosus, 
which do not sleep, circumnutate to an extraordinary extent, 
making many ellipses in the course of the day. 

Cytistis (Tribe 2), Trigonella and Medicago (Tribe 3). — Only 

Fig. 139. 





A. B. 

Medicago marina ; A. leaves during the day ; B, leaves asleep at night. 



a few observations were made on these three genera. The 
petioles on a young p'ant, about a foot in height, of Cyfisus 
fragrans rose at night, on one occasion 23° and on another 33°. 
The three leaflets als> bend upwards, and at the same time 



Chap. VII. SLEEP OF LEAVES. 345 

approach each other, so that the base of the central leaflet 
overlaps the bases of the two lateral leaflets. They bend 
up so nmch that they press against the stem; and on looking 
down on one of these 3 oung plants from vertically above, the 
lower surfaces of the leaflets are visible; and thus their upper 
surfaces, in accordance with the general rule, are best protected 
from radiation. Whilst the leaves on these young plants were 
thus behaving, those on an old bush in full flower did not sleep 
at night. 

Tiigonella Cretica resembles a Melilotus in its sleep, which will 
be immediately described. According to M. Eoyer,* the leaves 
of Medicago maculata rise up at night, and " se renversent un 
peu cle maniere a presenter obliquement au ciel leur face in- 
ferieure." A drawing is here given (Fig. 139) of the leaves 
of M. marina awake and asleep ; and this would almost serve 
for Oytisus fragrans in the same two states. 

Melilotus (Tribe 3).— The species in this gemis sleep in a 
remarkable manner. The three leaflets of each leaf twist through 
an angle of 90°, so that their blades stand vertically at night 
with one lateral erlge presented to the zenith (Fig. 140). We 
shall best understand the other and more complicated move- 
ments, if we imagine ourselves always to hold the leaf with the 
tip of the terminal leaflet pointed to the north. The leaflets in 
becoming vertical at night could of course twist so that their 
upper surfaces should face to either side ; but the two lateral 
leaflets always twist so that this surface tends to face the north, 
but as they move at the same time towards the terminal leaflet, 
the upper surface of the one faces about N.N.W., and that of 
the other N.N.E. The terminal leaflet behaves differently, for 
it twists to either side, the upper surface facing sometimes east 
and sometimes west, but rather more commonly west than east. 
The terminal leaflet also moves in another and more remarkable 
manner, for whilst its blade is twisting and becoming vertical, 
the whole leaflet bends to one side, and invariably to the side 
towards which the upper surface is directed; so that if this 
surface faces the west the whole leaflet bends to the west, until 
it comes into contact with the upper and vertical surface of 
the western lateral leaflet. Thus the upper surface of the 
terminal and of one of the two lateral leaflets is well protected. 

The fact of the terminal leaflet twisting indifferently to either 



Annales des Sc. Nat. Bot.' (5th series), ix. 1868, p. 368. 



346 



MODIFIED CIRCUMNUTATION. 



Chap. VII 



side and afterwards bending to the same side, seemed to us so 
remarkable, that we endeavoured to discover the cause. We 
imagined that at the commencement of the movement it might 
be determined by one of the two halves of the leaflet being 
a little heavier than the other. Therefore bits of wood were 
gummed on one side of several leaflets, but this produced no 
effect; and they continued to twist in the same direction as 



Fisr. 140. 




Melilotus officinalis : A, leaf during the daytime. B, another leaf asleep. 
C, a leaf asleep as viewed from vertically above ; but in this case the 
terminal leaflet did not happen to be in such close contact with the 
lateral one, as is usual. 

they had previously done. In order to discover whether the 
same leaflet twisted permanently in the same direction, black 
threads were tied to 20 leaves, the terminal leaflets of which 
twisted so that their upper surfaces faced west, and 14 white 
threads to leaflets which twisted to the east. These were ob- 
served occasionally during 14 days, and they all continued, with 
a single exception:, to twist aud bend in the same direction; for 



Chap. YJI. SLEEP OF LEAVES. 347 

one leaflet, which had originally faced east, was observed after 

9 days to face west. The seat of both the twisting and bending 
movement is in the pulvinus of the sub-petioles. 

We believe that the leaflets, especially the two lateral ones, 
in performing the above described complicated movements 
generally bend a little downwards ; but we are not sure of this, 
for, as far as the main petiole is concerned, its nocturnal move- 
ment is largely determined by the position which the leaf 
happens to occupy during the day. Thus one main petiole was 
observed to rise at night 59°, whilst three others rose only 7° 
and 9°. The petioles and sub-petioles are continually circum- 
nutating during the whole 24 h., as we shall presently see. 

The leaves of the following 15 species, M. officinalis, suaveolens, 
parviflora, alba, inftsta, dtntata, gracilis, sulcata, elegans, ccerulea, 
petitpurreatia, macrorrhiza, Itulici, secundiflora, and Taurica, 
sleep in nearly the same manner as just described; but the 
bending to one side of the terminal leaflet is apt to fail unless 
the plants are growing vigorously. With M.petitpierreara and 
sccundiflora the terminal leaflet was rarely seen to bend to one 
side. In young plants of M. Halica it bent in the usual manner, 
but with old plants in full flower, growing in the same pot and 
observed at the same hour, viz., 8.30 p.m., none of the terminal 
leaflets on several scores of leavt s had bent to one side, though 
they stood vertically ; nor nad the two lateral leaflets, though 
standing vertically, moved towards the terminal one. At 

10 30 p.m , and again one hour after midnight, the terminal 
leaflets had become very slightly bent to one side, and the 
lateral leaflets had mo ?ed a very little towards the terminal one, 
so that the posit on of the leaflets even at this late hour was far 
from the ordinary one. Again, with M. Taurica the terminal 
leaflets were never seen to bend towards either of the two lateral 
leaflets, chough these, whilst becoming vertical, had bent towards 
the terminal one. The sub-petiole of the terminal leaflet in 
this species is of unusual length, and if the leaflet had bent to 
one side, its upper surface could have come into contact only 
with the apex of either lateral leaflet ; and this, perhaps, is the 
meaning of the loss of the lateral movement. 

The cotyledons do not sleep at night. The first leaf consists of 
a single orbicular haflet, which twists at night so that the blade 
stands vertically. It is a remarkable fact that with M. Taurica, 
and in a somewhat less degree with M. macrorrhiza and petit- 
piemana, all the many small and young leaves produced during 



318 MODIFIED CIRCUMNUTATION. Chap. VU 

the early spring from shoots on some cut-down plants in the 
greenhouse, slept in a totally different manner from the normal 
one ; for the three leaflets, instead of twisting on their own axes 
so as to present their lateral edges to the zenith, turned upwards 
and stood vertically with their apices pointing to the zenith. 
They thus assumed nearly the same position as in the allied 
genus Trifolium ; and on the same principle that embryological 
characters reveal the lines of descent in the animal kingdom, so 
the movements of the small leaves in the above three species of 
Melilotus, perhaps indicate that this genus is descended from 
a form which was closely allied to and slept like a Trifolium. 
Moreover, there is one species, M. me sinensis, the leaves of 
which, on full-grown plants between 2 and 3 feet in height, 
sleep like the fort-going small leaves and like those of a Trifolium. 
We were so much surprised at this latter case that, until the 
flowers and fruit were examined, we thought that the seeds of 
some Trifolium had been sown by mistake instead of those of a 
Melilotus. It appears therefore probable that M. messanensis 
has either retained or recovered a primordial habit. 

The circumnutation of a leaf of M. officinalis was traced, 
the stem being left free; and the apex of the terminal leaflet 
described three laterally extended ellipses, between 8 a.m. and 
4 p.m.; after the latter hour the nocturnal twi-ting movement 
commenced. It was afterwards ascertained that the above 
movement was compounded of the circumnutation of the stem 
on a small scale, of the main petiole which moved most, and of 
the sub-petiole of the to-minal leaflet. The main petiole of a 
leaf having been secured to a stick, close to the base of the sub- 
petiole of the terminal leaflet, the latter described two small 
ellipses between 10.30 a.m., and 2 p.m. At 7.15 p.m., after th's 
same leaflet (as well as another) had twisted themselves into 
their vertical nocturnal position, they began to rise slowly, and 
continued to do so until 10.35 p.m., after which hour they were 
no longer observed. 

As M. messanensis sleeps in an anomalous manner, unlike that 
of any other species in the genus, the circumnutation of a 
terminal leaflet, with the stem secured, was traced during two 
days. On each morning the leaflet fell, until about noon, and 
then began to rise very slowly; but on the first clay the rising 
movement was interrupted between 1 and 3 p.m. by the formation 
of a laterally extended ellipse, and on the second day, at the 
game time, by two smaller ellipses. The rising movement then 



Chap. VII. SLEEP OF LEAVES. 349 

recommenced, and btcame rapid late in the evening, when 
the leaflet was beginning to go to sleep. The awaking or 
sinking movement had already commenced by 6.45 a.m on both 
mornings. 

Trifolium (Tribe 3). — The nyctitropic movements of 11 
species were observed, and were found to be closely similar. If 
we select a leaf of T. rcpens having an upright petiole, and with 
the three leaflets expanded horizontally, the two lateral leaflets 
will be seen in the evening to twist and approach each other, 
until their upper surfaces come into contact. At the same time 
they bend downwards in a plane at right angles to that of their 
former position, until their midribs form an angle of about 45° 
with the upper part of the petiole. This peculiar change of 
position requires a considerable amount of torsion in the pul- 
vinus. The terminal leaflet merely rises up without any twist- 

Fi 2 . 141. 





A. B. 

Trifolium repens: A, leaf during the day; B, leaf asleep at night. 

ing, and bends over until it rests on and forms a roof over the 
edges of the now vertical and united lateral leaflets. Thus the 
termiual leaflet always passes through an angle of at least 90°, 
generally of 130° or 140°, and not rarely — as was often observed 
with T. s bterraneum — of 180°. In this latter case the terminal 
leaflet stands at night horizontally (as in Fig. 141), with its 
lower surface fully exposed to the zenith. Besides the difference 
in the angles, at which the terminal leaflets stand at night in 
the individuals of the same species, the degree to which the 
lateral leaflets approach each other often likewise differs. 

We have seen that the cotyledons of some species and not of 
others rise up vertically at night. The first true leaf is generally 
unifoliate and orbicular ; it always rises, and either stands verti- 
cally at night or more commonly bends a little over so as to expose 
the lower surface obliquely to the zenith, in the same manner 
as does the terminal leaflet of the mature leaf. But it does not 
twist itself like the corresponding first simple leaf of Melilotus. 



350 MODIFIED CIKCUMNUTATION. Chap. VII 

With T. Pannonicum the first true leaf was generally unifoliate, 
but sometimes trifoliate, or again partially lobed and in an 
intermediate condition. 

Oircumnutation. — Sachs described in 1863* the spontaneous 
up and down movements of the leaflets of T. incarnatum, when 
kept in darkness. Pfeffer made many observations on the 
similar movements in T. pratensej He states that the terminal 
leaflet of this species, observed at different times, passed through 
angles of from 30° to 120° in the course of from Is to 4h. We 
observed the movements of T. subterraneum, resiipinatum, and 
repens. 

TrifoMvm subterraneum. — A petiole was secured close to the 
base of the three leaflets, and the movement of the terminal 
leaflet was traced during 26£ h., as shown in the figure on the 
next page 

Between 6.45 a.m. and 6 p.m. the apex moved 3 times up 
and 3 times down, completing 3 ellipses in 11 h. 15 m. The 
ascending and descending lines stand nearer to one another 
than is usual with most plants, yet there was some lateral 
motion. At 6 p.m. the great nocturnal rise commenced, and 
on the next morning the sinking of the leaflet was continued 
until 8.30 a.m., after which hour it circumnutated in the manner 
just described. In the figure the great nocturnal rise and 
the morning fall are greatly abbreviated, from the want of 
space, and are merely represented by a short curved line. The 
leaflet stood horizontally when at a point a little beneath the 
middle of the diagram; so that during the daytime it oscillated 
almost equally above and beneath a horizontal position. At 
8.30 a.m. it stood 48° beneath the horizon, and by 11.30 a.m. it 
had risen 50° above the horizon ; so that it passed through 98° 
in 3 h. By the aid of the tracing we ascertained that the 
distance travelled in the 3 h. by the apex of this leaflet was 
1*03 inch. If we look at the figure, and prolong upwards in 
our mind's eye the short curved broken line, which repre- 
sents the nocturnal course, we see that the latter movement is 
merely an exaggeration or pro^ngation of one of the diurnal 
ellipses. The same leaflet had been observed on the previous 
day, and the course then pursued was almost identically the 
same as that here described. 



* 'Flora,' 18H3, p. 497. 

t ' Die Period. Bewegungen,' 1875, pp. 35, 52. 



Chap. VII. 



SLEEP OF LEAVES. 



351 



Fig. 142. 






Trifolium resnpinatum. — A plant left entirely 
before a north-east win- 
dow, in such a position 
that a terminal leaflet 
projected at right angles 
to the source of the light, 
the sky being uniformly 
clouded all day. The 
movements of this leaflet 
were traced during two 
days, and on both were 
closely similar. Those 
executed on the second 
day are shown in Fig. 
143. The obliquity of 
the several lines is due 
partly to the manner in 
which the leaflet was 
viewed, and partly to its 
having moved a little to- 
wards the light. From 
7.50 a.m. to 8.40 a.m. the 
leaflet fell, that is, the 
awakening movement was 
continued. It then rose 
and moved a little late- 
rally towards the light. 
At 12.30 it retrograded, 
and at 2.30 resumed its 
original course, having 
thus completed a small 
ellipse during the middle 
of the day. In the even- 
ing it rose rapidly, and 
by 8 a.m. on the following 
morning had returned to 
exactly the same spot as 
on the previous morning. 
The line representing the 
nocturnal course ought 
to be extended much 
higher up, and is here 
abbreviated into a short, 



fre«. was placed 




352 



MODIFIED CIRCUMNUTATION. 



Chap. VIL 




H 



curved, broken line. The terminal leaflet, therefore, of this 
species described during the daytime only a single additional 

ellipse, instead of two ad- 
Fig. 143. ditional ones, as in the 
case of T. subterrat/eum. 
But we should remember 
that it was shown in the 
fourth chapter that the 
stem circumnutates, as no 
doubt does the main petiole 
and the sub-petioles; so 
that the movement repre- 
sented in fig. 143 is a com- 
pounded one We tried 
to observe the movements 
of a leaf kept during the 
day in darkness, but it 
,., .. ~ • j • began to go to sleep after 

infolium resupinatum : circumnutation ~ ° f 

and nyetitropic movements of the ter- <* n - J- 5 m -> anci tnis was 
minaf leaflet during 24 hours. well pronounced after 4 h. 

30 m. 

Trifolium repens. — A stem was secured close to the base of 
a moderately old leaf, and the movement of the terminal leaflet 
was observed during two days. This case is interesting solely 
from the simplicity of the movements, in contrast with those of 
the two preceding species. On the first day the leaflet fell 
between 8 a.m. and 3 p.m., and on the second between 7 a.m. 
and 1 p.m. On both days the descending course was somewhat 
zigzag, and this evidently represents the circumnutating move- 
ment of the two previous species during the middle of the day. 
After 1 p.m., Oct. 1st (Fig. 144), the leaflet began to rise, but 
the movement was slow on both days, both before and after 
this hour, until 4 p.m. The rapid evening and nocturnal rise 
then commenced. Thus in this species the course during 24 h. 
consists of a single great ellipse ; in T. resupinatum of two 
ellipses, one of which includes the nocturnal movement and is 
much elongated; and in T. subterrane^m of three ellipses, of 
which the nocturnal one is likewise of great length. 

Secur igera coronilla (Tribe 4). — The leaflets, which stand 
opposite one another and are numerous, rise up at night, come 
into close contact, a»«d bend backwards at a moderate angle 
towards the base of the petiole. 



Chap VII. 



SLEEP OF LEAVES. 



353 



Fig. 144. 



Lotus (Tribe 4).— The nyctitropic movements of 10 species 
in this genus were observed, and found to be the same. The 
main petiole rises a little at night, and 
the three leaflets rise till they become 
vertical, and at the same time approach 
each other. This was conspicuous with 
L. Jacobceus, in which the leaflets are 
almost linear. In most of the species 
the leaflets rise so much as to press 
against the stem, and not rarely they 
become inclined a little inwards with 
their lower surfaces exposed obliquely 
to the zenith. This was clearly the 
case with L. major, as its petioles are 
unusually long, and the leaflets are thus 
enabled to bend further inwards. The 
young leaves on the summits of the 
stems close up at night so much, as 
often to resemble large buds. The 
stipule-like leaflets, which are often of 
large size, rise up like the other leaflets, 
and press against the stem (Fig. 145). 
All the leaflets of L. Gebelii, and pro- 
bably of the other species, are provided 
at their bases with distinct pulvini, of 
a yellowish colour, and formed of very 
small cells. The circumnutation of a 
terminal leaflet of L. perigrinvs (with 
the stem secured) was traced during 
two days, but the movement was so 
simple that it is not worth while to 
give the diagram. The leaflet fell 
slowly from the early morning till 
about 1 p.m. It then rose gradually 
at first, but rapidly late in the evening. 

It occasionally stood still for about 20 m. during the day, and 
sometimes zigzagged a little. The movement of one of the 
basal, stipule-like leaflets was likewise traced in the same 
manner and at the same lime, and its course was closely similar 
to that of the terminal leaflet. 

In Tribe 5 of Bentham and Hooker, the sleep-movements 
of species in 12 genera have been observed by ourselves and 




Trifolium repens : circum- 
nutation and nyctitropic 
movements of a nearly 
full - grown terminal 
leaflet, traced on a ver- 
tical glass from 7 A.M. 
Sept. 30th to 8 A.M. Oct. 
1st. Nocturnal course, 
represented by curved 
broken line, much ab- 
breviated. 



354 



MODIFIED CIKCUMNUTATION. 



Chat VII 



others, but only in Robinia with any care. Vsoralea acaulu 
raises its three leaflets at night ; whilst Amorpha fruticosa* 
Balea alop.curoid.s, and Indigofera tinctoria depress them. 
Duchartre f states that Ttphrosia caribcea is the sole example 
>f " folioles couchees le long du petiole et vers la base ; " but a 

Fig. 145. 




A. B. 

Lotus deticus : A, stem with leaves awake during the day ; B, with leaves 
asleep at night. SS, stipule-like leaflets. 

similar movement occurs, as we have already seen, and shall 
again see in other cases. Wistaria Sinensis, according to 
Eoyer.J " abaisse les folioles qui par une disposition bizarre 
sont inclinees dans la meme feuille, les sup6rieures vers le 



* Ducharte, ' Elements 
BAanique, 1867, p. 31"J. 
t Ibid., p. 347. 



de 



X 'Ann. dee Sciences, Nats, 
Bot.' C5th series), ix. 1868. 



Chap. VII. 



SLEEP OF LEAVES. 



355 



sommet, les inferieures vers la base du petiole cornmun ; " but 
the leaflets on a young plant observed by us in -the green- 
house merely sank vertically downwards at night. The leaflets 
are raised in Sphcerophysa salsola, Colutea arborea, and Astra- 
galus uliginosus, but are depressed, according to Linnaeus, in 
Glycyrrhiza. The leaflets of Robinia pst-vdo-acacia likewise sink 
vertically down at night, but the petioles rise a little, viz., in 
one case 3°, and in another 4°. The circumnutating move- 
ments of a terminal leaflet on a rather old leaf were traced 
during two days, and were simple. The leaflet fell slowly, in a 
slightly zigzag line, from 8 a.m. to 5 p.m., and then more 
rapidly ; by 7 a.m. on the following morning it had risen to its 
diurnal position. There was only one peculiarity in the move- 
ment, namely, that on both days there was a distinct though 
small oscillation up and down between 8.80 and 10 a.m., and 
this would probably have been more strongly pronounced if 
the leaf had been younger. 

Cor on ilia rosea (Tribe 6). — The leaves bear 9 or 10 pairs of 
opposite leaflets, which during the day stand horizontally, with 

Fig. 146. 




Coronilla rosea : leaf asleep. 

their midribs at right angles to the petiole. At night they rise 
up, so that the opposite leaflets come nearly into contact, and 
those on the younger leaves into close contact. At the same 
time they bend back towards the base of the period, until their 
midribs form with it angles of from 40° to 50° in a vertical 
plane, as here figured (Fig. 146). The leaflets, however, some- 
times bend ?o much back that their midribs become parallel to 
and lie on the petiole. They thus occupy a reversed position 
to what they do in several Leguminosse, for instance, in Mimosa 



556 



MODIFIED CIECUMNUTATION. 



Chaf. VII. 



pudica ; but, from standing further apart, they do not overlap 
one another nearly so much as in this latter plant. The main 
petiole is curved slightly downwards during the day, but 
straightens itself at night. In three cases it rose from 3° above 
the horizon at noon, to 9° at 10 p.m. ; from 11° to 33° ; and from 
5° to 33° — the amount of angular movement in this latter case 
amounting to 28°. In several other species of Coronilla the 
leaflets showed only feeble movements of a similar kind. 

Jdedysarum coronarium (Tribe 6). — The small lateral leaflets 
on plants growing out of doors rose up vertically at night, but 
the large terminal one became only moderately inclined. The 
petioles apparently did not rise at all. 

Smithia Pfundii (Tribe 6). — The leaflets rise up vertically, 
and the main petiole also rises considerably. 

Araehis hypogcea (Tribe 6;.— The shape of a leaf, with its two 
pairs of leaflets, is shown at A (Fig. 147) ; and a leaf asleep, 

Fig. 147. 




Araehis hypogcea: A, leaf during the day, seen from vertically above ; B, 
leaf asleep, seen laterally; copied from a photogr.iph. Figures much 
reduced. 



traced from a photograph (made by the aid of aluminium 
light), is given at B. The two terminal leaflets twist round at 
night until their blades stand vertically, and approach each 
other until they meet, at the same time moving a little upwards 
and backwards. The two lateral leaflets meet each other in the 
same manner, but move to a greater extent forwards, that is, in 
a contrary direction to the two terminal leaflets, which they 
partially embrace. Thus all four leaflets form together a single 
packet, with their edges directed to the zenith, and with their 
lower surfaces turned outwards. On a plant which was not 
growing vigorously the closed leaflets seemed too heavy for the 



Chap. VII. 



SLEEP OF LEAVES. 



357 






petioles to support them in a vertical position, so that each 
night the main petiole became twisted, and all the packets were 
extended horizontally, with the lower surfaces of the leaflets on 
one side directed to the zenith in a most anomalous manner. 
This fact is mentioned solely as a caution, as it surprised us 
greatly, until we discovered that it was an anomaly. The 
petioles are inclined upwards during the day, but sink at night, 
so as to stand at about right angles with the stem. The amount 
of sinking was measured only on one occasion, and found to be 
39°. A petiole was secured to a stick at the base of the two 
terminal leaflets, and the circumnutating movement of one of 
these leaflets was traced from 6.40 a.m. to 10.40 p.m., the plant 
being illuminated from above. The temperature was 17° — 17^° C, 
and therefore rather too low. During the 16 h. the leaflet moved 
thrice up and thrice down, and as the ascending and descend- 
ing lines did not coincide, three ellipses were formed. 

Desmodium gyrans (Tribe 6). — A large and full-grown leaf of 
this plant, so famous for the spontaneous 
movements of the two little lateral leaflets, 
is here represented (Fig. 148). The large 
terminal leaflet sleeps by sinking vertically 
down, whilst the petiole rises up. The coty- 
ledons do not sleep, but the first -formed leaf 
sleeps equally well as the older ones. The 
appearance presented by a sleeping branch 
and one in the day-time, copied from two 
photographs, are shown at A and B (Fig. 
149), and we see how at night the leaves are 
crowded together, as if for mutual pro- 
tection, by the rising of the petioles. The 
petioles of the younger leaves near the sum- 
mits of the shoots rise up at night, so as to 
stand vertical and parallel to the stem ; 
whilst those on the sides were found in four 
cases to have risen respectively 461°, 36°, 
20°, and 19*5° above the inclined positions 
which they had occupied during the day. 
For instance, in the first of these four cases 
the petiole stood in the day at 23°, and at 
night at 695° above the horizon. In the 
evening the rising of the petioles is almost 
completed before the leaflets sink perpendicularly downwards. 




Desmodium qyrans: 
leaf seen from 
above, reduced 
to one-half na- 
tural size. The 
minute stipulei 
unusually large 



358 



MODIFIED CIRC [IMPUTATION 



Chap. VU. 



Circumvutation. — The circumnutating movements of four 
young shoots were observed during 5 h. 15 m. ; and in this time 
each completed an oval figure of small size. The main petiole 
also circumnutates rapidly, for in the course of 31m. (temp, 
91° F.) it changed its course by as much as a rectangle six times, 
describing a figure which apparently, represented two ellipses. 



Fig. 149 




Desmodium gyrans : A, stem during the day ; B, stem with leaves asleep. 
Copied from a photograph ; figures reduced. 

The movement of the terminal leaflet by means of its sub- 
petiole or pulvinus is quite as rapid, or even more so, than that 
of the main petiole, and has much greater amplitude. Pfeffer 
has reen* these leaflets move through an angle of 8° in the 
course of from 10 to 30 seconds. 

A fine, nearly full-grown leaf on a young plant, 8 inches in 
height, with the stem secured to a stick at the base of the leaf, 
vas observed from 8.30 a.m. June 22nd to 8 a.m. June 24th 



Die Period. Beweg.,' p. 35. 



Chap. VII. SLEEP OF LEAVES. 359 

In the diagram given on the next page (Fig. 150), the two 
curved broken lines at the base, which represent the nocturnal 
courses, ought to be prolonged far downwards. On the first 
day the leaflet moved thrice down and thrice up, and to a con- 
siderable distance laterally ; the course was also remarkably 
crooked. The dots were generally made every hour; if they 
had been made every few minutes all the lines would have been 
zigzag to an extraordinary degree, with here and there a loop 
formed. We may infer that this would have been the caee, 
because five dots were made in the course of 31m. (between 
12.34 and 1.5 p.m.), and we see in the upper part of the diagram 
how crooked the course here is: if only the first and last dots 
had been joined we should have had a straight line. Exactly 
the same fact may be seen in the lines representing the course 
between 2.24 p.m. and 3 p.m., when six intermediate dots were 
made ; and again at 4.46 and 4.50. But the result was widely 
different after 6 p.m., — that is, after the great nocturnal descent 
had commenced ; for though nine dots were then made in the 
course of 32 m , when these were joined (see Figure) the line thus 
formed was almost straight. The leaflets, therefore, begin to 
descend in the afternoon by zigzag lines, but as soon as the 
descent becomes rapid their whole energy is expended in thus 
moving, and their course becomes rectilinear. After the leaflets 
are completely asleep they move very little or not at all. 

Had the above plant been subjected to a higher temperature 
than 67° — 70° F., the movements of the terminal leaflet would 
probably have been even more rapid and wider in extent than 
those shown in the diagram ; for a plant was kept for some time 
in the hot-houee at from 92° — 93° F., and in the course of 35 m. 
the apex of a leaflet twice descended and once ascended, travelling 
over a space of 1*2 inch in a vertical direction and of "82 inch in 
a horizontal direction. Whilst thus moving the leaflet also 
rotated on its own axis (and this was a point to which no atten- 
tion had been before paid), for the plane of the blade differed by 
41° after an interval of only a few minutes. Occasionally the 
leaflet stood still for a short time. There was no jerking move- 
ment, which is so characteristic of the little lateral leaflets. A 
sudden and considerable fall of temperature causes the terminal 
leaflet to sink downwards ; thus a cut-off leaf was immersed in 
water at 95° F., which was slowly raised to 103° F., and after- 
wards allowed to sink to 70° F., and the sub-petiole of the ter- 
minal leaflet then curved downwards. The water was afterwards 
24 



660 MODIFIED CIRCUMNUTATION. 

Fig. 1.50. 



Chap YH 



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Chap. VII. SLEEP OF LEAVES 361 

raised to 120° F., and the sub-petiole straightened itself. Similar 
experiments with leaves in water were twice repeated, with 
nearly the same result. It should be added, that water raised 
to even 122° F. does not soon kill a leaf. A plant was placed 
in darkness at 8.37 a.m., and at 2 p.m. (i.e. after 5 h. 23 m.), though 
the leaflets had sunk considerably, they had by no means ac- 
quired their nocturnal vertically dependent position. Pfeffer, on 
the other hand, says * that this occurred with him in from f h. 
to 2 h. ; perhaps the difference in our results may be due to 
the plant on which we experimented being a very young and 
vigorous seedling. 

The Movements of the little Lateral Leaflets. — These have been so 
often described, that we will endeavour to be as brief as possible 
in giving a few new facts and conclusions. The leaflets some- 
times quickly change their position by as much as nearly 180° ; 
and their sub-petioles can then be seen to become greatly curved. 
They rotate on their own axes, so that their upper surfaces are 
directed to all points of the compass. The figure described by 
the apex is an irregular oval or ellipse. They sometimes re- 
main stationary for a period. In these several respects there is 
no difference, except in rapidity and extent, between their move- 
ments and the lesser ones performed by the large terminal 
leaflet whilst making its great oscillations. The movements of 
the little leaflets are much influenced, as is well known, by 
temperature. This was clearly shown by immersing leaves with 
motionless leaflets in cold water, which was slowly raised to 
103° F., and the leaflets then moved quickly, describing about a 
dozen little irregular circles in 40 m. By this time the water 
had Income much cooler, and the movements became slower or 
almost ceased ; it was then raised to 100° F., and the leaflets 
again began to move quickly. On another occasion a tuft of 
fine leaves was immersed in water at 53° F., and the leaflets 
were of course motionless. The water was raised to 99°, and 
the leaflets soon began to move ; it was raised to 105°, and the 
movements became much more rapid ; each little circle or oval 
being completed in from 1 m. 30 s. to 1 m. 45 s. There was, 
however, no jerking, and this fact may perhaps be attributed to 
the resistance of the water. 

Sachs stat»*s that the leaflets do not move until the surround- 
ing air is as high as 71° — 72° F., and this agrees with our 



* * Die Period. Beweg.,' p. 39. 



362 MODIFIED CIKCUMNUTATION. Chap. VII. 

experience on full-grown, or nearly full-grown, plants. But the 
leaflets of young seedlings exhibit a jerking movement at much 
lower temperatures. A seedling was kept (April 16th) in a room 
for half the day where the temperature was steady at 64° F., 
and the one leaflet which it bore was continually jerking, but 
not so rapidly as in the hot-house. The pot was taken in the 
evening into a bed-room where the temperature remained at 
62° during nearly the whole night; at 10 and 11p.m. and at 
1 a.m. the leaflet was still jerking rapidly ; at 3.30 a m. it was not 
seen to jerk, but was observed during only a short time. It was, 
however, now inclined at a much lower angle than that occupied 
at 1 a.m. At 6.30 a.m. (temp. 61° F.) its inclination was still 
less than before, and again less at 6.45 a.m. ; by 7.40 A m. it had 
risen, and at 8.30 a.m. was again seen to jerk. This leaflet, 
therefore, was moving during the whole night, and the move- 
ment was by jerks up to 1 a.m. (and possibly later) and again at 
8.30 am., though the temperature was only 61° to 62° F. "We 
must therefore conclude that the lateral leaflets produced by 
young plants differ somewhat in constitution from those on 
older plants. 

In the large genus Desmodium by far the greater number 
of the species are trifoliate ; but some are unifoliate, and even 
the same plant may bear uni- and trifoliate leaves. In most 
of the species the lateral leaflets are only a little smaller than 
the terminal one. Therefore the lateral leaflets of D. gyrans 
(see former Fig. 148) must be considered as almost rudi- 
mentary. They are also rudimentary in function, if this ex- 
pression may be used ; for they certainly do not sleep like the 
full-sized terminal leaflets. It is, however, possible that the 
sinking down of the leaflets between 1 a.m. and 6.45 a.m , as 
above described, may represent sleep. It is well known that 
the leaflets go on jerking during the early part of the night; 
but my gardener observed (Oct. 13th) a plant in the hot-house 
between 5 and 5.30 a.m., the temperature having been kept up 
to 82° F., and found that all the leaflets were inclined, but he 
saw no jerking movement until 6.55 a.m., by which time the 
terminal leaflet had risen and was awake. Two days after- 
wards (Oct. 15th) the same plant was observed by him at 
4.47 a.m. (temp. 77° F.), and he found that the large terminal 
leaflets were awake, though not quite horizontal ; and the only 
cause which we could assign for this anomalous wakefulness was 
that the plant had been kept for experimental purposes during 



Chap. VII. SLEEP OF LEAVES. 363 

the previous day at an unusually high temperature ; the little 
lateral leaflets were also jerking at this hour, but whether 
there was any connection between this latter fact and the sub- 
horizontal position of the terminal leaflets we do not know. 
Anyhow, it is certain that the lateral leaflets do not sleep like 
the terminal leaflets; and in so far they may be scid to be 
in a functionally rudimentary condition They are in a similar 
condition in relation to irritability; for if a plant be shaken 
or syringed, the terminal leaflets sink down to about 45° be- 
neath the horizon ; but we could never detect any effect thus 
produced on the lateral leaflets; yet we are not prepared to 
assert positively that rubbing or pricking the pulvinus produces 
no effect. 

As in the case of most rudimentary organs, the leaflets are 
variable in size ; they often depart from their normal position 
and do not stand opposite one another; and one of the two is 
frequently absent. This absence appeared in some, but not in 
all the cases, to be due to the leaflet having become completely 
confluent with the main petiole, as might be inferred from the 
presence of a slight ridge along its upper margin, and from the 
course of the vessels. In one instance there was a vestige of 
the leaflet, in the shape of a minute point, at the further end of the 
ridge. The frequent, sudden, and complete disappearance of one 
or both of the rudimentary leaflets is a rather singular fact ; but 
it is a much more surprising one that the leaves which are first 
developed on seedling plants are not provided with them. Thus, 
on one seedling the seventh leaf above the cotyledons was the 
first which bore any lateral leaflets, and then only a single one. 
On another seedling, the eleventh leaf first bore a leaflet ; of the 
nine succeeding leaves five bore a single lateral leaflet, and 
Jour bore none at all ; at last a leaf, the twenty-first above the 
cotyledons, was provided with two rudimentary lateral leaflets. 
Irom a widespread analogy in the animal kingdom, it might 
have been expected that these rudimentary leaflets would have 
been better developed and more regularly present on very young 
than on older plants. But bearing in mind, firstly, that long- 
lost characters sometimes reappear late in life, and secondly, 
that the species of Desmodium are generally trifoliate, but that 
some are unifoliate, the suspicion arises that D. gyrans is 
descended from a unifoliate species, and that this was descended 
from a trifoliate one ; for in this case both the absence of the 
little lateral leaflets. on very young seedlings, and their sub- 



364 MODIFIED CTRCUMNUTATION Chap. VII 

sequent appearance, may be attributed to reversion to more 01 
less distant progenitors.* 

No one supposes that the rapid movements of the lateral 
leaflets of D. gyrans are of any use to the plant; and why 
they should behave in this manner is quite unknown. We 
imagined that their power of movement might stand in some 
relation with their rudimentary condition, and therefore ob- 
served the almost rudimentary leaflets of Mimosa albida vel 
sensitiva (of which a drawing will hereafter be given, Fig. 159) ; 
but they exhibited no extraordinary movements, and at night 
they went to sleep like the full-sized leaflets. There is, how- 
ever, this remarkable difference in the two cases ; in Desmo- 
dium the pulvinus of the rudimentary leaflets has not been 
reduced in length, in correspondence with the reduction of the 
blade, to the same extent as has occurred in the Mimosa ; and it 
is on the length and degree of curvature of the pulvinus that the 
amount of movement of the blade depends. Thus, the average 
length of the pulvinus in the large terminal leaflets of Desmo- 
dium is 3 mm., whilst that of the rudimentary leaflets is 2'86 mm. ; 
so that they differ only a little in length. But in diameter they 
differ much, that of the pulvinus of the little leaflets being only 
0*3 mm. to 0*4 mm.; whilst that of the terminal leaflets is 
1-33 mm. If we now turn to the Mimosa, we find that the 
average length of the pulvinus of the almost rudimentary 
leaflets is only - 466 mm., or rather more than a quarter of the 
length of the pulvinus of the full-sized leaflets, namely, 1 • 66 mm. 
In this small reduction in length of the pulvinus of the rudi- 
mentary leaflets of Desmodium, we apparently have the proxi- 
mate cause of their great and rapid circumnutating movement, 
in contrast with that of the almost rudimentary leaflets of the 
Mimosa. The small size and weight of the blade, and the little 
resistance opposed by the air to its movement, no doubt also come 
into play ; for we have seen that these leaflets if immersed in 
water, when the resistance would be much greater, were pre- 
vented from jerking forwards. Why, during the reduction of 
the lateral leaflets of Desmodium, or during their reappearance 
— if they owe their origin to reversion — the pulvinus should 
have been so much less affected than the blade, whilst with the 



* Desmodium vezpertilionis is rudimentary lateral leaflets. Du- 
closely allied to D. gyrans, and chartre, 'Eie'meutsde Botanique, 
it seems only occasionally to bear 1867, p. 353. 



Chap. VII. 



SLEEP OF LEAVES. 



365 



Mimosa the pulvinus has been greatly reduced, we do not 
know. Nevertheless, it deserves notice that the reduction of 
the leaflets in these two genera has apparently been effected by 
a different process and for a different end ; for with the Mimosa 
the reduction of the inner and basal leaflets was necessary from 
the want of space; but no such necessity exists with Desmo- 
dium, and the reduction of its lateral leaflets seems to have 
been due to the principle of compensation, in consequence of 
the great size of the terminal leaflet. 

Uraria (Tribe 6) and Centro&ema (Tribe 8). — The leaflets of 
Uraria lay opus and the leaves of a Centrosema from Brazil 
both sink vertically down at night. In the latter plant the 
petiole at the same time rose 16 £°. 

Amphkarpcea monoica (Tribe 8). — The leaflets sink down ver- 
tically at night, and the petioles likewise fall considerably. 




Amphicarpcea monoica : circumnutation and nyctitropic movement of leaf 
during 48 h. ; its apex 9 inches from the vertical glass. Figure reduced 
to one-third of original scale. Plant illuminated from above; temp. 
I7i°-18i° C. 

A petiole, which was carefully observed, stood during the day 
25° above the horizon and at night 32° below it ; it therefore 
fell 57°. A filament was fixed transversely across the terminal 
leaflet of a fine young leaf (2 J inches in length including the 



'SS6 MODIFIED CIRCUMNUTATION. Chap. VII. 

petiole), and the movement of the whole leaf was traced on a 
vertical glass. This was a bad plan in some respects, because 
the rotation of the leaflet, independently of its rising or falling, 
raised and depressed the filament ; but it was the best plan for 
our special purpose of observing whether the leaf moved much 
after it had gone to sleep. The plant had twined closely round 
a thin stick, so that the circumnutation of the stem was pre- 
vented. The movement of the leaf was traced during 48 h., 
from 9 a.m. July 10th to 9 a.m. July 12th. In the figure given 
(Fig. 151) we see how complicated its course was on both days : 
during the second day it Changed its course greatly 13 times. 
The leaflets began to go to sleep a little after 6 p.m., and by 
7.15 p.m. hung vertically down and were completely asleep; 
but on both nights they continued to move from 7.15 p.m. 
to 10.40 and 10.50 p.m., quite as much as during the day ; and 
this was the point which we wished to ascertain. "We see in 
the figure that the great sinking movement late in the evening 
does not differ essentially from the circumnutation during 
the day. 

Glycine hispida (Tribe 8). — The three leaflets sink vertically 
down at night. 

Erythrina (Tribe 8). — Five species were observed, and the 
leaflets of all sank vertically down at night ; with E. caffra and 
with a second unnamed species, the petioles at the same time 
rose slightly. The movements of the terminal leaflet of E. crista- 
galli (with the main petiole secured to a stick) were traced 
from 6.40 a.m., June 8th, to 8 a.m. on the 10th. In order to 
observe the nyctitropic movements of this plant, it is necessary 
that it should have grown in a warm greenhouse, for out of 
doors in our climate it does not sleep. We see in the tracing 
(Fig. 152) that the leaflet oscillated twice up and down between 
early morning and noon ; it then fell greatly, afterwards rising 
till 3 p.m. At this latter hour the great nocturnal fall com- 
menced. On the second day (of which the tracing is not given) 
there was exactly the same double oscillation before noon, but 
only a very small one in the afternoon. On the third morning 
the leaflet moved laterally, which was due to its beginning to 
assume an oblique position, as seems invariably to occur with 
the leaflets of this species as they grow old. On both nights after 
the leaflets were asleep and hung vertically down, they continued 
to move a little both up and down, and from side to side. 

Erythrina caffra. — A filament was fixed transversely across 



Chap. VII. 



SLEEP OF LEAVES. 



367 



a terminal leaflet, as we wished 
to observe its movements when 
asleep. The plant was placed 
in the morning of June lUth 
under a skylight, where the 
light was not bright ; and we 
do not know whether it was 
owing to this cause or to the 
plant having been disturbed, 
but the leaflet hung vertically 
down all day; nevertheless it 
circumnutated in this posi- 
tion, describing a figure which 
represented two irregular el- 
lipses. On the next day it 
circumnutated in a greater 
degree, describing four irre- 
gular ellipses, and by 3 p.m. 
had risen into a horizontal po- 
sition. By 7.15 p.m. it was 
asleep and vertically depen- 
dent, but continued to circum- 
nutate as long as observed, 
until 11 p.m. 

Erythrina corallodendron. — 
The movements of a terminal 
leaflet were traced. During 
the second day it oscillated 
four times up and four times 
down between 8 a.m. and 4 
p.m., after which hour the great 
nocturnal fall commenced. On 
the third day the movement 
was equally great in ampli- 
tude, but was remarkably 
simple, for the leaflet rose in 
an almost perfectly straight 
line from 6.50 a.m. to 3 p.m., 
and then sank down in an 
equally straight line until 
vertically dependent and 



6'40a.m.8 i tt 



2'j>.m.> 



G°J5'a.m.9 t t 



3 Q 



Erythrina crista-galli : circumnuta- 
tion and nyctitropic movement 
of terminal leaflet, 3f inches in 
length, traced during 25 h. ; apex 
of leaf 3 J inches from the vertical 
glass. Figure reduced to one-half 
of original scale. Plant illumi- 
nated from above: temp. 17A°- 
18£° C. 



#68 MODIFIED CIRCUMNUTATION. Chap. TO. 

Apios tuherosa (Tribe 8). — The leaflets sink vertically dowc 
at night. 

Phaseolus vulgaris (Tribe 8). — The leaflets likewise sink verti- 
cally down at night. In the greenhouse the petiole of a young 
leaf rose 16°, and that of an older leaf 10° at night. With 
plants growing ont of doors the leaflets apparently do not sleep 
until somewhat late in the season, for on the nights of July 11th 
and 12th none of them were asleep ; whereas on the night of 
August 15th the same plants had most of their leaflets verti- 
cally dependent and asleep. With Ph. caiacalla and Eernan- 
dtsii, the primary unifoliate leaves and the leaflets of the 
secondary trifoliate leaves sink vertically down at night. This 
holds good with the secondary trifoliate leaves of Ph. Eox- 
ourghii, but it is remarkable that the primary unifoliate leaves, 
which are much elongated, rise at night from about 20° to 
about 60° above the horizon. With older seedlings, however, 
having the secondary leaves just developed, the primary leaves 
stand in the middle of the day horizontally, or are deflected 
a little beneath the horizon. In one such case the primary 
leaves rose from 26° beneath the horizon at noon, to 20° above 
it at 10 p.m.; whilst at this same hour the leaflets of the 
secondary leaves were vertically dependent. Here, then, we 
have the extraordinary case of the primary and secondary 
leaves on the same plant moving at the same time in opposite 
directions. 

We have now seen that the leaflets in the six genera of Pha- 
seoleae observed by us (with the exception of the primary leaves 
of Phaseolus Roxburyhii) all sleep in the same manner, namely, 
by sinking vertically down. The movements of the petioles 
were observed in only three of these genera. They rose in 
Centrosema and Phaseolus, and sunk in Amphicarpsea. 

Sophora chrysojahylla (Tribe 10). — The leaflets rise at night, 
and are at the same time directed towards the apex of the leaf, 
as in Mimosa pudica. 

Ccesalpinia, ffcematoxylon, Oltditschia, Poinciana. — The leaflets 
of two species of Csesalpinia (Tribe 13) rose at night. With 
Hozmatoxylon Campechianum (Tribe 13) the leaflets move for- 
wards at night, so that their midribs stand parallel to the 
petiole, and their now vertical lower surfaces are turned out- 
wards (Fig. 153). The petiole sinks a little. In Gleditschia, if 
we understand correctly Duchartre's description, and in Poin* 



Chai\ VII. 



SLEEP OF LEAVES. 



3G9 



ciana Gilliesii (both belonging to Tribe 13), the leaves behave 
in the same manner. 

Fig. 153. 




Hcematoxylon Campechianum : A, branch during daytime ; B, branch with 
leaves asleep, reduced to two-thirds of natural scale. 

Cassia (Tribe 14). — The nyctitropic movements of the leaves 
in many species in this genus are closely alike, and are highly 
complex. They were first briefly described by Linnseus, and since 
by Duchartre. Our observations were made chiefly on 0. flori- 
bunda * and corymbosa, but several other species were casually 
observed. The horizontally extended leaflets sink down verti- 
cally at night ; but not simply, as in so many other genera, for 
each leaflet rotates on its own axis,, so that its lower surface 
faces outwards. The upper surfaces of the opposite leaflets are 
thus brought into contact with one another beneath the petiole, 
and are well protected (Fig. 154). The rotation and other move- 
ments are effected by means of a well-developed pulvinus at the 
base of each leaflet, as could be plainly seen when a straight 
narrow black line had been painted along it during the day. 
The two terminal leaflets in the daytime include rather less than 
a right angle; but their divergence increases greatly whilst they 



* I am informed by Mr. Dyer 
that Mr. Bentham believes that 
C. floribunda (a common green- 
house bush) is a hybrid raited in 
France, and that it cunes very 



near to C, Icevigatn. It is no doubt 
the same as the form described by 
Lindley (' Bot. Keg.,' Tab. 1422; 
*»« C. Nerhertiana. 



370 



MODIFIED CIKCUMNUTATION. 



Chap. VIL 



sink downwards and rotate, so that they stand laterally at night, 
as may be seen in the figure. Moreover, they move somewhat 
backwards, so as to point towards the base of the petiole. 

Fig. 154. 




Cassia corymbcsa: A, plant during day; B, same jlant at night. 
Both figures copied from photographs. 



lu one instance we found that the midrib of a terminal 
leaflet formed at night an angle of 36°, with a line dropped 



Chap. VII. 



SLEEP OF LEAVES. 



371 



perpendicularly from the end of the petiole. The second pair 
of leaflets likewise moves a little backwards, but less than the 
terminal pair; and the third pair moves vertically downwards, 
or even a little forwards. Thus all the leaflets, in those species 
which bear only 3 or 4 pairs, tend to form a single packet, with 
their upper surfaces in contact, and their lower surfaces turned 
outwards. Lastly, the main petiole rises at night, but with 
leaves of different ages to very different degrees, namely, some 
rose through an angle of only 12°, and others as much as 41°. 

Cassia call iautha.— The leaves bear a large number of leaflets, 
which move at night in nearly the same manner as just 
described; but the petioles apparently do not rise, and one 
which was carefully observed certainly fell 3°. 

Cassia pubeaceits. — The chief difference in the nyctitropic 

Fig. 155. 




Caseia pubescens; A, upper part of plant during the day ; B, same pant 
at night. Figures reduced from photographs. 

movements of this species, compared with those of the former 
species, consists in the leaflets not rotating nearly so much; 



372 MODIFIED CIRCUMNUTATION. Chap VII 

therefore their lower surfaces face but little outwards at night. 
The petioles, which during the day are inclined only a little 
above the horizon, rise at night in a remarkable manner, and 
stand neirly or quite vertically. This, together with the 
dependent position of the leaflets, makes the whole plant won- 
derfully compact at night. In the two foregoing figures, copied 
from photographs, the same plant is represented awake and 
asleep (Fig. 155), and we see how different is its appearance. 

Cassia rnimosuides — At night the numerous leaflets on each 
leaf rotate on their axes, and their tips move towards the apex 
of the leaf; they thus become imbricated with their lower 
surfaces directed upwards, and with their midribs almost 
parallel to the petiole. Consequently, this species differs from 
all the others seen by us, with the exception of the following 
one, in the leaflets not sinking down at night. A petiole, the 
movement of which was measured, rose 8° at night. 

Cassia Barclayana. — The leaflets of this Australian species are 
numerous, very narrow, and almost linear. At night they rise up 
a little, and also move towards the apex of the leaf. For instance, 
two opposite leaflets which diverged from one another during 
the day at an angle of 104°, diverged at night only 72° ; so that 
each had risen 16° above its diurnal position. The petiole of a 
young leaf rose at night 34°, and that of an older leaf 19°. 
Owing to the slight movement of the leaflets and the consider- 
able movement of the petiole, the bush presents a different 
appearance at night to what it does by day; yet the leaves can 
hardly be said to sleep. 

The circumnutating movements of the leaves of C. floribunda, 
calliantlia, and pubesceas were observed, each during three or four 
days ; they were essentially alike, those of the last-named species 
being the simplest. The petiole of C. floribunda was secured to 
a stick at the base of the two terminal leaflets, and a filament 
was fixed along the midrib of one of them. Its movements were 
traced from 1 p.m. on August 13th to 8.30 a.m. 17th ; but those 
during the last 2 h. are alone given in Fig. 156. From 8 a.m. on 
each day (by which hour the leaf had assumed its diurnal posi- 
tion) to 2 or 3 p.m., it either zigzagged or circumnutated over 
nearly the same small space; at between 2 and 3 p.m. the great 
evening fall commenced. The lines representing this fall and 
the early morning rise are oblique, owing to the peculiar manner 
in which the leaflets sleep, as already described. After the 
leaflet was asleep at 6 p.m., and whilst the glass filament hung 



Chap. VII 



SLEEP OF LEAVES. 



373 




£^~3 

? £ 2 



V -SB 



41 



^ 






a «o 

^ br,co 



nr 



S bo E 



perpenlicularly down, the movement of its apex was traced 
until 10.30 p.m.; and during this whole time it swayed from 
side to side, completing more than one ellipse. 

Bauhiuia (Tribe 15).— Fig. 156 

The nyctitropic movements 
of four species were alike, 
and were highly peculiar. 
A plant raised from seed 
sent us from South Brazil 
by Fritz Miiller, was more 
especially observed. The 
loaves are large and deeply 
notched at their ends. At 
night the two halves rise 
up and close completely 
together, like the opposite 
leaflets of many Legumi- 
nosse. With very young 
plants the petioles rise con- 
siderably at the same time ; 
one, which was inclined at 
noon 45° above the hori- 
zon, at night stood at 75° ; 
it thus rose 30°; another 
rose 34°. Whilst the two 
halves of the leaf are closing, 
the midrib at first sinks 
vertically downwards and 
afterwards bends back- 
wards, so as to pass close 
along one side of its own 
upwardly inclined petiole; 
the midrib being thus di- 
rected towards the stem or 
axis of the plant. The angle 
which the midrib formed 
with the horizon was mea- // js 

sured in one case at dif- 
ferent hours: at noon it stood horizontally; late in the even- 
ing it depended vertically; then rose to the opposite side, and 
at 10.15 p.m. stood at only 27° beneath the horizon, being 
directed towards the stem. It had thus travelled through 153° 



"Z £ bo 
o o 5 



a O g 

III 



5 5 "3 <S 



S3 



^ O c3 S 

r2 S U * 

5 " c <** 



374 MODIFIED CIRCUMNUTATIOiN. Chap. VII. 

Owing to this movement — to the leaves being folded — and to 
the petioles rising, the whole plant is as much more compact at 
night than during the day, as a fastigiate Lombardy poplar is 
compared with any other species of poplar. It is remarkable 
that when our plants had grown a little older, viz., to a hoight 
of 2 or 3 feet, the petioles did not rise at night, and the midribs 
of the folded leaves were no longer bent back along one side of 
the petiole. We have noticed in some other genera that the 
petioles of very young plants rise much more at night than do 
those of older plants. 

Tamarindus Indica (Tribe 16). — The leaflets approach or 
meet each other at night, and are all directed towards the apex 
of the leaf. They thus become imbricated with their midribs 
parallel to the petiole. The movement is closely similar to 
that of Hsematoxylon (see former Fig. 153), but more striking 
from the greater number of the leaflets. 

Adenanthera, Prosopis, and Neptunia (Tribe 20). — With Ade- 
nanthera paronia the leaflets turn edgeways and sink at night. 
In Prosopis they turn upwards With Neptunia oleracea the 
leaflets on the opposite sides of the same pinna come into 
contact at night and are directed forwards. The pinnse them- 
selves move downwards, and at the same time backwards or 
towards the stem of the plant. The main petiole rises. 

Mimosa pudica (Tribe 20). — This plant has been the subject of 
innumerable observations; but there are some points in rela- 
tion to our subject which have not been sufficiently attended 
to. At night, as is well known, the opposite leaflets come into 
contact and point towards the apex of the leaf ; they thus be- 
come neatly imbricated with their upper surfaces protected. The 
four pinnse also approach each other closely, and the whole leaf 
is thus rendered very compact. The main petiole sinks down- 
wards during the day till late in the evening, and rises until 
very early in the morning. The stem is continually circumnu- 
tating at a rapid rate, though not to a wide extent. Some very 
young plants, kept in darkness, were observed during two days, 
and although subjected to a rather low temperature of 57° — 59° F., 
the stem of one described four small ellipses in the course ot 
12 h. We shall immediately see that the main petiole is like- 
wise continually circumnutating, as is each separate pinna and 
each separate leaflet. Therefore, if the movement of the apex 
of any one leaflet were to be traced, the course described would 
be compounded of the movements of four separate parts. 



Chap. VII. 



SLEEP OF LEAVES. 



375 



A filament had been fixed on the previous eveiing, longi- 
tudinally to the main petiole of a nearly full-grown, highly- 
sensitive leaf (four inches in length), the stem having been 
secured to a stick at its base ; and a tracing was made on a 
vertical glass in the hot-bouse under a high temperature. In 
the figure given (Fig. 157), tbe 



first dot was made at 8.30 a.m. 
August 2nd, and the last at 7 
p.m. on the 3rd. During 12 h. on 
the first day the petiole moved 
thrice downwards and twice 
upwards. Within the same 
length of time on the second 
day, it moved five times down- 
wards and four times upwards. 



Fig. 157. 



8°30'a.m.2^- 



I'SftunAR* 



ing lines do not coincide, the 
petiole manifestly circumnu- 
tates; the great evening fall 
and nocturnal rise being an 
exaggeration of one of the cir • 
cumnutations. It should, how- 
ever, be observed that the pe- 
tiole fell much lower down in 
the evenings than could be 
seen on the vertical glass or is 
represented in the diagram. 
After 7 p.m. on the 3rd (when 
the last dot in Fig. 157 was 
made) the pot was carried into 
a bed-room, and the petiole was 
found at 12.50 a.m. (i.e. after 
midnight) standing almost up- 
right, and much more highly 
inclined than it was at 10.40 
p.m. When observed again at 
4 a.m. it had begun to fall, and 
continued falling till 6.15 a.m., 
after which hour it zigzagged and again circumnutatecl. Similar 
observations were made on another petiole, with nearly the 
same result. 
On two other occasions the movement of the main petiole 

25 




e'p.m.2™ * 



Mimosa pudica : circunmutation and 
nventropic movement of main pe- 
tiole, traced during 34 h. 30 m. 



376 MODIFIED CIKCUMNUTATION. Chap. VII 

was observed every two or three minutes, the plants being kept 
at a rather high temperature, viz., on the first occasion at 
77° — 81° F., and the filament then described 2\ ellipses in 69 m. 
On the second occasion, when the temperature was 81° — 86° F., 
it made rather more than 3 ellipses in 67 m. Therefore, 
Fig. 157, though now sufficiently complex, would have been in- 
comparably more so, if dots had been made on the glass every 
2 or 3 minutes, instead of every hour or half-hour. Although 
the main petiole is continually and rapidly describing small 
ellipses during the day, yet after the great nocturnal rising 
movement has commenced, if dots are made every 2 or 3 
minutes, as was done for an hour between 9.30 and 10.30 p.m. 
(temp. 84° F.), and the dots are then joined, an almost abso- 
lutely straight line is the result. 

To show that the movement of the petiole is in all proba- 
bility due to the varying turgescence of the pulvinus, and not 
to growth (in accordance with the conclusions of Pfeffer), a very 
old leaf, with some of its leaflets yellowish and hardly at all 
sensitive, was selected for observation, and the plant was kept 
at the highly favourable temp, of 80° F. The petiole fell from 
8 a.m. till 10.15 a.m., it then rose a little in a somewhat zigzag 
line, often remaining stationary, till 5 p.m., when the great 
evening fall commenced, which was continued till at least 
10 p.m. By 7 a.m. on the following morning it had risen to the 
same level as on the previous morning, and then descended in 
a zigzag line. But from 10.30 a.m. till 4.15 p.m. it remained 
almost motionless, all power of movement being now lost. The 
petiole, therefore, of this very old leaf, which must have long 
ceased growing, moved periodically ; but instead of circum- 
nntating several times during the day, it moved only twice 
down and twice up in the course of 24 h., with the ascending 
and descending lines not coincident. 

It has already been stated that the pinnae move independently 
of the main petiole. The petiole of a leaf was fixed to a cork 
support, close to the point whence the four pinnae diverge, with 
a short fine filament cemented longitudi Dally to one of the two 
terminal pinnae, and a graduated semicircle was placed close 
beneath it. By looking vertically down, its angular or lateral 
movements could be measured w T ith accuracy. Between noon 
and 4.15 r m. the pinna changed its position to one side by only 
7°; but not continuously in the same direction, as it moved 
four times to one side, and three times to the opposite side, 



Chap. VII. SLEEP OF LEAVES. 377 

in one instance to the extent of 16°. This pinna, therefore, 
circumnutated. Later in the evening the four pinnae approach 
each other, and the one which was observed moved inwards 
59° between noon and 6.45 p.m. Ten observations were made 
in the course of 2 h. 20 m. (at average intervals of 14 m.) 
between 4.25 and 6.45 p.m. ; and there was now, when the leat 
was going to sleep, no swaying from side to side, but a steady 
inward movement. Here therefore there is in the evening the 
same conversion of a circumnutating into a steady movement 
in one direction, as in the case of the main petiole. 

It has also been stated that each separate leaflet circum- 
nutates. A pinna was cemented with shellac on the summit ot 
a little stick driven firmly into the ground, immediately beneath 
a pair of leaflets, to the midribs of both of which excessively 
fine glass filaments were attached. This treatment did not 
injure the leaflets, for they went to sleep in the usual manner, 
and long retained their sensitiveness. The movements of one 
of them were traced during 49 h., as shown in Fig. 158. On the 
first day the leaflet sank down till 11.30 a.m., and then rose 
till late in the evening in a zigzag line, indicating circum- 
nutation. On the second day, when more accustomed to its 
new state, it oscillated twice up and twice down during the 
24 h. This plant was subjected to a rather low temperature, 
viz., 62° — 64° F. ; had it been kept warmer, no doubt the move- 
ments of the leaflet would have been much more rapid and 
complicated. It may be seen in the diagram that the ascending 
and descending lines do not coincide; but the large amount of 
lateral movement in the evening is the result of the leaflets 
bending towards the apex of the leaf when going to sleep. 
Another leaflet was casually observed, and found to be con- 
tinually circumnutating during the same length of time. 

The circumnutation of the leaves is not destroyed by their 
being subjected to moderately long continued darkness ; but the 
proper periodicity of their movements is lost. Some very young 
seedlings were kept during two days in the dark (temp. 57° — 59° 
F.), except when the circumnutation of their stems was occa- 
sionally observed ; and on the evening of the second day the 
leaflets did not fully and properly go to sleep. The pot was 
then placed for three days in a dark cupboard, under nearly the 
same temperature, and at the close of this period the leaflets 
showed no signs of sleeping, and were only slightly sensitive to 
a touch. On the following day the stem was cemented to a 



378 



MODIFIED CIECUMNUTATION. 



Chap. Vll 



stick, and the movements of two leaves were traced on a vertical 
glass during 72 h. The plants were still kept in the dark, ex- 
cepting that at each observation, which lasted 3 or 4 minutes, 




UttilS** 



jOHS&aM 



Mimosa pudica; circumnutation and nyctitropic movement of a leaflet 
(with pinna secured), traced on a vertical glass, from 8 A.M. Sept. 14th 
to 9 A.M. 16th. 



they were illuminated by two candles. On the third day the 
leaflets still exhibited a vestige of sensitiveness when forcibly 
pressed, but in the evening they showed no signs of sleep. 
Nevertheless, their petioles continued to circumnutate distinctly, 



Chap. VII. 



SLEEP OF LEAVES. 



379 



although the proper order of their movements in relation to tht 
day and night was wholly lost. Thus, one leaf descended during 
the first two nights (i.e. between 10 p.m. and 7 a.m. next morn- 
ing) instead of ascending, and on the third night it moved 
chiefly in a lateral direction. The second leaf behaved in an 
equally abnormal manner, moving laterally during the first 
night, descending greatly during the second, and ascending to 
an unusual height during the third night. 

With plants kept at a high temperature and exposed to the 
light, the most rapid circumnutating movement of the apex 
of a leaf which was observed, amounted to -^ of an inch in 
one second; and this would have equalled £ of an inch in a 
minute, had not the leaf occasionally stood still. The actual 
distance travelled by the apex (as ascertained by a measure 
placed close to the leaf) was on one occasion nearly | of an inch 
in a vertical direction in 15 m. ; and on another occasion f of an 
inch in 60 m. ; but there was also some lateral movement. 

Mimosa albida* — The leaves of this plant, one of which is here 
figured (Fig. 159) reduced to § of the natural size, present some 

Fig. 159. 




Mimosa albida ; leaf seen from vertically above. 

interesting peculiarities. It consists of a long petiole bearing 
only two pinnae (here represented as rather more divergent 
than is usual), each with two pairs of leaflets. But the inner 



* Mr. Thistleton Dyer informs Linn. Soc.,' vol. xxx. p. 390) to 

us that this Peruvian plant (which' be " the species or variety which 

was sent to us from Kew) is con- most commonly represents the M 

sidered by Mr. Bentham (' Trans. sensitiva of our gardens." 



380 MODIFIED CD3CUMNUTATI0N. Chap. VII. 

basal leaflets are greatly reduced in size, owing probably to the 
want of space for their full development, so that they may be 
considered as almost rudimentary. They vary somewhat in 
size, and both occasionally disappear, or only one. Neverthe- 
less, they are not in the least rudimentary in function, for they 
are sensitive, extremely heliotropic, circum nutate at nearly the 
same rate as the fully developed leaflets, and assume when 
asleep exactly the same position. JWith $■• pudica the inner 
leaflets at the base and between the pinnce are likewise much 
shortened and obliquely truncated ; this fact was well seen in 
some seedlings of M. pudica, in which the third leaf above the 
cotyledons bore only two pinnae, each with only 3 or 4 pairs of 
leaflets, of which the inner basal one was less than half as long 
as its fellow; so that the whole leaf resembled pretty closely 
that of M. albida. In this latter species the main petiole termi- 
nates in a little point, and on each side of this there is a pair 
of minute, flattened, lancet-shaped projections, hairy on their 
margins, which drop off and disappear soon after the leaf is 
fully developed. There can hardly be a doubt that these little 
projections are the last and fugacious representatives of an 
additional pair of leaflets to each pinna; for the outer one is 
twice as broad as the inner one, and a little longer, viz. -^ of an 
inch, whilst the inner one is only f=£ long. Now if the basal 
pair of leaflets of the existing leaves were to become rudimen- 
tary, we should expect that the rudiments would still exhibit 
some trace of their present great inequality of size. The con- 
clusion that the pinnae of the parent-form of M. albida possessed 
at least three pairs of leaflets, instead of, as at present, only two, 
is supported by the structure of the first true leaf; for this 
consists of a simple petiole, often bearing three pairs of leaflets. 
This latter fact, as well as the presence of the rudiments, both 
lead to the conclusion that M. albida is descended from a form 
the leaves of which bore more than two pairs of leaflets. The 
second leaf above the cotyledons resembles in all respects the 
leaves on fully developed plants. 

When the leaves go to sleep, each leaflet twists half round, 
so as to present its edge to the zenith, and comes into close 
contact with its fellow. The pinnse also approach each other 
closely, so that the four terminal leaflets come together. The 
large basal leaflets (with the little rudimentary ones in contact 
with them) move inwards and forwards, so as to embrace the 
outside of the united terminal leaflets, and thus all eight leaflets 



Chm\ VII. SLEEP OF LEAVES. 381 

(the rudimentary ones included) form together a single vertical 
packet. The two pinnae at the same time that they approach 
each other sink downwards, and thus instead of extending hori- 
zontally in the same line with the main petiole, as during the 
day, they depend at night at about 45°, or even at a greater 
angle, beneath the horizon. The movement of the main petiole 
seems to be variable ; we have seen it in the evening 27° lower 
than during the day ; but sometimes in nearly the same position. 
Nevertheless, a sinking movement in the evening and a rising 
one during the night is probably the normal course, for this 
was well-marked in the petiole of the first-formed true leaf. 

The circumnutation of the main petiole of a young leaf was 
traced during 21 days, and was considerable in extent, but less 
complex than that of M. pudica. The movement was much 
more lateral than is usual with circumnutating leaves, and this 
was the sole peculiarity which it presented. The apex of 
one of the terminal leaflets was seen under the microscope to 
travel -^ of an inch in 3 minutes. 

Mimosa marginata. — The opposite leaflets rise up and approach 
each other at night, but do not come into close contact, except in 
the case of very young leaflets on vigorous shoots. Full-grown 
leaflets circumnutate during the day slowly and on a small scale. 

Schrankia uncinata (Tribe 20). — A leaf consists of two or three 
pairs of pinnae, each bearing many small leaflets. These, when 
the plant is asleep, are directed forwards and become imbricated. 
The angle between the two terminal pinnae was diminished at 
night, in one case by 15°; and they sank almost vertically down- 
wards. The hinder pairs of pinnae likewise sink downwards, 
but do not converge, that is, move towards the apex of the leaf. 
The main petiole does not become depressed, at least during the 
evening. In this latter respect, as well as in the sinking of the 
} innae, there is a marked difference between the nyctitropic 
movements of the present plant and of Mimosa pudica. It 
should, however, be added that our specimen was not in a very 
vigorous condition. The pinnae of Schrankia acuhata also sink 
at night. 

Acacia Farnesiana (Tribe 22). — The different appearance pre- 
sented by a bush of this plant when asleep and awake is won- 
derful. The same leaf in the two states is shown in the following 
figure (Fig. 160). The leaflets move towards the apex of the 
pmna and become imbricated, and the pinnae then look like bits 
of dangling string. The following remarks and measurements 



382 



MODIFIED CIRCUMNUTATION. 



Chap. Vll 



do not fully apply to the small leaf here figured. The pinnae 
move forwards and at the same time sink downwards, whilst 
the main petiole rises considerably. With respect to the degree 
of movement : the two terminal pinnae of one specimen formed 
together an angle of 100° during the day, and at flight of only 
38°, so each had moved 31° forwards. The penultimate pinnae 
during the day formed together an angle of 180°, that is, they 
stood in a straight line opposite one another, and at night each 
had moved 65° forwards. The basal pair of pinnae were directed 

Fig. 160. 




A. B. 

Acacia Farnesiana; A, leaf during the day; B, the same leaf at night. 

during the day, each about 21° backwards, and at night 38° 
forwards, so each had moved 59° forwards. But the pinnae at 
the same time sink greatly, and sometimes hang almost perpen- 
dicularly downwards. The main petiole, on the other hand, 
rises much : by 8.30 p.m. one stood 34° higher than at noon, 
and by 6.40 a.m. on the following morning it was still higher 
by 10°; shortly after this hour the diurnal sinking move- 
ment commenced. The course of a nearly full-grown leaf was 
traced during 14 h ; it was strongly zigzag, and apparently 



CW. YIl. SLEEP OF LEAVES. 38d 

represented five ellipses, with their longer axes differently 
directed. 

Albizzia lopliantha (Tribe 23). — The leaflets at night come into 
contact with one another, and are directed towards the apex of 
the pinna. The pinnae approach one another, but remain in the 
same plane as during the day ; and in this respect they differ 
much from those of the above Schrankia and Acacia. The main 
petiole rises but little. The first-formed leaf above the coty- 
ledons bore 11 leaflets on each side, and these slept like those 
on the subsequently formed leaves ; but the petiole of this first 
leaf was curved downwards during the day and at night 
straightened itself, so that the chord of its arc then stood 16° 
higher than in the day-time. 

Melaleuca ericcefolia (Myrtacese). — According to Bouche (' Bot. 
Zeit.,' 1874, p. 359) the leaves sleep at night, in nearly the same 
manner as those of certain species of Pimelia. 

(Enothtra mollissima (Onagrarieae). — According to Linnaeus 
( c Somnus Plantarum '), the leaves rise up vertically at night. 

Passiflora gracilis (Passifloracae). — The young leaves sleep by 
their blades hanging vertically downwards, and the whole length 
of the petiole then becomes somewhat curved downwards. 
Externally no trace of a pulvinus can be seen. The petiole of 
the uppermost leaf on a young shoot stood at 10.45 a.m. at 33° 
above the horizon ; and at 10.30 p.m., when the blade was verti- 
cally dependent, at only 15°, so the petiole had fallen 18°. That 
of the next older leaf fell only 7°. From some unknown cause 
the leaves do not always sleep properly. The stem of a plant, 
which had stood for some time before a north-east window, was 
secured to a stick at the base of a young leaf, the blade of 
which was inclined at 40° below the horizon. From its position 
the leaf had to be viewed obliquely, consequently the vertically 
ascending and descending movements appeared when traced 
oblique. On the first day (Oct. 12th) the leaf descended in a 
zigzag line until late in the evening; and by 8.15 a.m. on the 
13th had risen to nearly the same level as on the previous 
morning. A new tracing was now begun (Fig. 161). The 
leaf continued to rise until 8.50 a.m., then moved a little to the 
right, and afterwards descended. Between 11 a.m. and 5 p.m. it 
circumnutated, and after the latter hour the great nocturnal 
fall commenced. At 7.15 p.m. it depended vertically. The 
dotted line ought to have been prolonged much lower down in 
the figure. By 6.50 a.m. on the following morning (14th) the 



381 



MODIFIED CIKCUMNUTATION. 



Chap. VII. 



leaf had risen greatly, and continued to rise till 7.50 a.m., aftei 
which hour it redescended. It should be observed that the lines 
traced on this second morning would have coincided with and 
confused those previously traced, had not the pot been slided 
a very little to the left. In the evening (14th) a mark was 
placed behind the filament attached to the apex of the leaf, and 
its movement was carefully traced from 5 p.m. to 10.15 p.m. 

Fig. 161. 




Passiflora gracilis: circumnutation and nyctitropic movement of leaf 
traced on vertical glass, from 8.20 A.M. Oct. i3th to 10 A.M. 14th 
Figure reduced to two-thirds of original scale. 

Between 5 and 7.15 p.m. the leaf descended in a straight line; 
and at the latter hour it appeared vertically dependent. But 
between 7.15 and 10.15 p.m. the line consisted of a succession 
of steps, the cause of which we could not understand ; it was, 
however, manifest that the movement was no longer a simple 
descending one. 

Siegesbeckia orien talis (Compositse). — Some seedlings were 
raised in the middle of winter and kept in the hot-house ; they 
flowered, but did not grow well, and their leaves never showed 
any signs of sleep. The leaves on other seedlings raised in May 
wore horizontal at noon (June 22nd), and depended at a consi* 



Chap. VII. 



SLEEP OF LEAVES. 



38, f 



derable angle beneath the horizon at 10 p.m. In the case of four 
youngish leaves, which were from 2 to 2j inches in length, 
these angles were found to be 50°, 56°, 60°, and 65°. At the 
end of August, when the plants had grown to a height of 10 to LI 
inches, the younger leaves were so much curved downwards at 
night that they might truly be said to be asleep. This is one 

Fig. 162. 




Nicotianct gla»ca : shoots with leaves expanded during the day, and asleep 
at night. Figures copied from photographs, and reduced. 



of the species which must be well illuminated during the day 
in order to sleep, for on two occasions when plants were kept 
all day in a room with north-east windows, the leaves did not 
sleep at night. The same cause probably accounts for the 
leaves on our seedlings raised in the dead of the winter not 
Bleeping. Professor Pfeffer informs us that the leaves oi 
another species (S. Joruttensis ?) hang vertically down at night. 



B86 



MODIFIED OIECUMN UTATIOX. 



Chap. VII 



iun5jp«kia^ 



W°JA'p.m.lO l . 




1/ oman cmrulea said purpurea (Convolvulaceae). —The leases on 
very young plants, a foot or two in height, are depressed at night 

to between 68° and 80° 
%- 163 - beneath the horizon; 

and some hang quite 
vertically downwards. 
On the following morn- 
ing they again rise into 
a horizontal position. 
The petioles become 
at night downwardly 
curved, either through 
their entire length or in 
the upper part alone ; 
and this apparently 
causes the depression 
of the blade. It seems 
necessary that the 
leaves should be well 
illuminated during the 
day in order to sleep, 
for those which stood 
on the back of a plant 
before a north-east 
window did not sleep. 

Nicotiana tabacum 
(var. Virginian) and 
glauca (Solaneae).— The 
young leaves of both 
these species sleep by 
bendinh vertically up- 
wards. Figures of two 
shoots of N. ftauca, 
aw^ake and asleep (Fig. 
162), are given on p 
385 : one of the shoots, 
from which the photo- 
graphs were taken, was 
accidentally bent to one 
side. 

At the base of the petiole of N. tabacum, on the outside, there 
is a mass of cells, which are rather smaller than elsewhere, and 



$£M'a.mJ3 f ! 1 



>5°15'ii.L 



J1L" 



»/&j 



JB l 



3'p.i 

Nicotiana tabacum : circumnutation and nyc- 
titropic movement of a leaf (5^- inches in 
length), traced on a vertical glass, from 
3 P.M. July 10th to 8.10 A.M. 13th. Apex 
of leaf 4 inches from glass. Temp. 17£°- 
I83 C. Figure reduced to one-half 
original scale. 



Chap. VII. SLEEP OF LEAVES. 387 

have their longer axes differently directed from the ceUs of the 
parenchyma, and may therefore be considered as forming a sort 
of pnlvinus. A young plant of N. tabacum was selected, and 
the circummitation of the fifth leaf above the cotyledons was 
observed during three days. On the first morning (July 10th) 
the leaf fell from 9 to 10 a.m., which is its normal course, but 
rose during the remainder of the day; and this no doubt was 
due to its being illuminated exclusively from above; for properly 
the evening rise does not commence until 3 or 4 p.m. In the 
figure as given on p. 386 (Fig. 163) the first dot wasjnade at 
3 p.m. ; and the tracing was continued for the following 65 h. 
When the leaf pointed to the dot next above that marked 3 p.m. 
it stood horizontally. The tracing is remarkable only from its 
simplicity and the straightness of the lines. The leaf each day 
described a single great ellipse ; for it should be observed that 
the ascending and descending lines do not coincide. On the 
evening of the 11th the leaf did not descend quite so low as 
usual, and it now zigzagged a little. The diurnal sinking move- 
ment had already commenced each morning by 7 a.m. The broken 
lines at the top of the figure, representing the nocturnal vertical 
position of the leaf, ought to be prolonged much higher up. 

Mirabilis longiflora and jalapa (Nyctagine?e). — The first pair 
of leaves above the cotyledons, produced by seedlings of both 
these species, were considerably divergent during the day, and 
at night stood up vertically in close contact with one another. 
The two upper leaves on an older seedling were almost horizontal 
by day, and at night stood up vertically, but were not in close 
contact, owing to the resistance offered by the central bud. 

Polygonum aviculare (Polygoneae). — Professor Batalin informs 
us that the young leaves rise up vertically at night. This is 
likewise the case, according to Linnseus, with several species 
of Amaranthus (Amaranthacese) ; and we observed a sleep move- 
ment of this kind in one member of the genus. Again, with 
Clienopodium album (Chenopodiese), the upper young leaves of 
some seedlings, about 4 inches in height, were horizontal or 
sub-horizontal during the day, and at 10 p.m. on Mqxch 7th 
were quite, or almost quite, vertical. Other seedlings raised in 
the greenhouse during the winter (Jan. 28th) were observed day 
and night, and no difference could be perceived in the position 
of their leaves. According to Bouche (' Bot. Zeitung/ 1874, 
p. 359; the leaves of Pimdia linoides and spectabilis (Thymeleae) 
sleep at night. 



388 MODIFIED CIRCUMNUTATION. Chap. VII. 

Euphorbia jacquiniceflora (Euphorbiacese). — Mr. Lynch 
called our attention to the fact that the yonng leaves of this 
plant sleep by depending vertically. The third leaf from the 
summit (March 1] th) was inclined during the day 30° beneath 
the horizon, and at night hung vertically down, as did some of 
the still younger leaves. It rose up to its former level on the 
following morning. The fourth and fifth leaves from the summit 
stood horizontally during the day, and sank down at night only 
38°. The sixth leaf did not sensibly alter its position. The 
sinking movement is due to the downward curvature of the 
petiole, no part of which exhibits any structure like that of 
a pulvinus. Early on the morning of June 7th a filament was 
fixed longitudinally to a young leaf (the third from the summit, 
and 2f inches in length), and its movements were traced on 
a vertical glass during 72 h., the plant being illuminated from 
above through a skylight. Each day the leaf fell in a nearly 
straight line from 7 a.m. to 5 p.m., after which hour it was so 
much inclined downwards that the movement could no longer 
be traced ; and during the latter part of each night, or early in 
the morning, the leaf rose. It therefore circumnutated in a 
very simple manner, making a single large ellipse every 24 h., 
for the ascending and descending lines did not coincide. On 
each successive morning it stood at a less height than on the 
previous one, and this was probably due, partly to the increasing 
age of the leaf, and partly to the illumination being insufficient ; 
for although the leaves are very slightly heliotropic, yet, accord- 
ing to Mr. Lynch's and our own observations, their inclination 
during the day is determined by the intensity of the light. On 
the third day, by which time the extent of the descending 
movement had much decreased, the line traced was plainly 
much more zigzag than on any previous day, and it appeared 
as if some of its powers of movement were thus expended. At 
10 p.m. on June 7th, when the leaf depended vertically, its move- 
ments were observed by a mark being placed behind it, and the 
end of the attached filament was seen to oscillate slowly and 
slightly from side to side, as well as upwards and downwards. 

PhyUanthus Mruri (Euphorbiacese). — The leaflets of this 
plant sleep, as described by Pfeffer,* in a remarkable manner, 
apparently like those of Cassia, for they sink downwards at 
uight and twist round, so that their lower surfaces are turned 



Die Period. Bewcg.,' p. 159. 



Chap. VII. SLEEP OF LEAVES 389 

outwards. They are furnished, as might have been expected 
from this complex kind of movement, with a pulvinus. 

Gymnospekms. 

Pinus Nordmatunana (Coniferse). — M. Chatin states * that the 
leaves, which are horizontal during the day, rise up at night, so 
as to assume a position almost perpendicular to the branch from 
which they arise ; we presume that he here refers to a horizontal 
branch. He adds : " En meme temps, ce mouvement direction 
est accompagne d'un mouvement de torsion imprime a la partie 
basilaire de la feuille, et pouvant sou vent parcourir un arc do 
90 degres." As the lower surfaces of the leaves are white, 
whilst the upper are dark green, the tree presents a widely 
different appearance by day and night. The leaves on a small 
tree in a pot did not exhibit with us any nyctitropic move- 
ments. We have seen in a former chapter that the leaves of 
Pinus pinaster and Austriaca are continually circumnutating. 

Monocotyledons. 

Thalia dealbata (Cannaceae). — The leaves of this plant sleep 
by turning vertically upwards ; they are furnished with a well- 
developed pulvinus. It is the only instance known to us of 
a very large leaf sleeping. The blade of a young leaf, which 
was as yet only 13i inches in length and 62 in breadth,- formed 
at noon an angle with its tall petiole of 121°, and at night stood 
vertically in a line with it, and so had risen 59°. The actual 
distance travelled by the apex (as measured by an orthogonic 
tracing) of another large leaf, between 7.30 a.m. and 10 p.m., was 
101 inches. The circumnutation of two young and dwarfed 
leaves, arising amongst the taller leaves at the base of the plant, 
was traced on a vertical glass during two days. On the first day 
the apex of one, and on the second day the apex of the other leaf, 
described between 6.40 a.m. and 4 pm. two ellipses, the longer 
axes of which were extended in very different directions from the 
lines representing the great diurnal sinking and nocturnal rising 
movement. 

Maranta arundinacea (Cannacese). — The blades of the leaves, 
which are furnished with a pulvinus, stand horizontally during 



* ' Comptes Rendus,' Jan. 1876, p. 171. 



390 MODIFIED CIRCUMNUTATION. Chap. VII 

the day or between 10° and 20° above the horizon, and at night 
vertically upwards. They therefore rise between 70° and 90° at 
night. The plant was placed at noon in the dark in the hot- 
house, and on the following day the movements of the leaves 
were traced. Between 8.40 and 10 30 a.m. they rose, and then 
fell greatly till 1.37 p.m. But by 3 p.m. they had again risen a 
little, and continued to rise during the rest of the afternoon and 
night ; on the following morning they stood at the same level as 
on the previous day. Darkness, therefore, during a day and a 
half does not interfere with the periodicity of their, movements. 
On a warm but stormy evening, the plant whilst being brought 
into the house, had its leaves violently shaken, and at night not 
one went to sleep. On the next morning the plant was taken 
back to the hot-house, and again at night the leaves did not 
sleep ; but on the ensuing night they rose in the usual manner 
between 70° and 80°. This fact is analogous with what we 
have observed with climbing plants, namely, that much agitation 
checks for a time their power of circumnutation ; but the effect 
in this instance was much more strongly marked and prolonged. 
Colocasia antiquorum {Caladium esculentum, Hort.) (Aroideae). 
— The leaves of this plant sleep by their blades sinking in the 
evening, so as to stand highly inclined, or even quite vertical'y 
with their tips pointing to the ground. They are not provided 
with a pulvinus. The blade of one stood at noon 1° beneath the 
' horizon; at 4.20 p.m., 20° ; at 6 p.m., 43° ; at 7.20 p.m., 69° ; and at 
8 30 p.m., 68° ; so it had now begun to rise ; at 10.15 p.m. it stood 
at 65°, and on the following early morning at 11° beneath the 
horizon. The circumnutation of another young leaf (with its 
petiole only 3j inches, and the blade 4 inches in length), was 
traced on a vertical glass during 48 h. ; it was dimly illuminated 
through a skylight, and this seemed to disturb the proper perio- 
dicity of its movements. Nevertheless, the leaf fell greatly 
during both afternoons, till either 7.10 pm. or 9 p.m., when it 
rose a little and moved laterally. By an early hour on both 
mornings, it had assumed its diurnal position. The well-marked 
lateral movement for a short time in the early part of the night, 
was the only interesting fact which it presented, as this caused 
the ascending and descending lines not to coincide, in accord- 
ance with the general rule with circumnutating organs. The 
movements of the leaves of this plant are thus of the most 
simple kind; and the tracing is not worth giving. We have 
seen that in another genus of the Aroideas, namely, Pistia, the 



Chap. VII. 



SLEEP OF LEAVES. 



191 



leaves rise so much at night that they may almost be said to 



Strephium floribundum* (Graminese). — The oval leaves are 
provided with a pulvinus, and are extended horizontally or 
declined a little beneath the horizon during the day. Those 
on tho upright culms simply rise up vertically at night, so 
that their tips are directed towards the zenith. (Fig. 164.) 

Fig. 164. 





Strephium floribundum : culms with leaves during the day, and when asl<?ej: 
at night. Figures reduced. 

Horizontally extended leaves arising from much inclined or 
almost horizontal culms, move at night so that their tips 
point towards the apex of the culm, with one lateral margin 
directed towards the zenith; and in order to assume this 
position the leaves have to twist on their own axes through an 
angle of nearly 90°. Thus the surface of the blade always stands 
vertically, whatever may be the position of the midrib or of the 
leaf as a whole. 

The circumnutation of a young leaf (2 ■ 3 inches in length) was 
traced during 48 h. (Fig. 165). The* movement was remarkably 
simple; the leaf descended from before 6.40 a.m. until 2 or 
2.50 p.m., and then rose so as to stand vertically at about 6 p.m., 
descending again late in the night or in the very early morning. 



* A. Brongniart first observed 
that the leaves of this plant and 
of Marsilea sleep : see ' Bull, de 

26 



la Soc. Bot. de France/ torn, vii 
I860, p. 470. 



392 



MODIFIED CIRCUMNUTATION. 



Chap. VII 



On the second day the descending line zigzagged slightly. As 
jr lg 165# usual, the ascending and de- 

scending lines did not coincide. 
) On another occasion, when the 

/ temperature was a little higher, 

f viz., 24°-26d° C, a leaf was 

observed 17 times between 8.50 
a.m. and 12.16 p.m. ; it changed 
its course by as much as a 
rectangle six times in this in- 
terval of 3 h. 26 m., and de- 
scribed two irregular triangles 
and a half. The leaf, therefore, 
on this occasion circumnutated 
.rapidly and in a complex 
manner. 



ACOTYLEDONS. 

Marsilea quadrifoliaia (Mar- 
siieaceas). — The shape of a leaf, 
expanded horizontally during 
the day, is shown at A (Fig. 166). 
Each leaflet is provided with 
a well-developed pulvinus. 
When the leaves sleep, the two 
terminal leaflets rise up, twist 
half round and come into con- 
tact with one another (B), and 
are afterwards embraced by the 
two lower leaflets (G); so that 
the four leaflets with their lower 
surfaces turned outwards form 
a vertical packet. The curva- 
ture of the summit of the petiole 
of the leaf figured asleep, is 
merely accidental. The plant 
was brought into a room, where 
the temperature was only a little 
above 60° F., and the movement 
of one of the leaflets (the petiole 
having been secured) was traced 




Strephium floribundum : circumnu- 
tation and nyctitropic movement 
of a leaf, traced from 9 A.M. June 
26th to 8.45 A.M. 27th ; filament 
fixed along the midrib. Apex of 
leaf 8.J inches from the vertical 
glass ; plant illuminated from 
above. Temp. 23£°-24£° C. 



Chap Ml 



SLEEP OF LEAVES. 



393 



during 24 h. (Fig. 167). The leaf fell from the early morning 
till 1.50 p.m., and then rose till 6 p.m., when it was asleep. A 

Fig. 166. 




A. B. C. 

Marsilca quadrifoliaia: A, leaf during the day, seen from vertically above 
B, leaf beginning to go to sleep, seen laterally ; C, the same asleep. 
Figures reduced to one-half of natural scale. 

vertically dependent glass filament was now fixed to one of the 
terminal and inner leaflets; and part of the tracing in Fig. 167, 
after 6 p.m., shows that it continued to sink,. making one zigzag, 
until 10.40 p.m. At 6.45 a.m. on the following morning, the leaf 
was awaking, and the filament pointed above the vertical glass, 




6° p.m. 



J.G11/ nm. 



Marsilea quadrifoliaia : circumnutation and nyctitropic movement of leaflet 
traced on vertical glass, during nearly 24 h. Figure reduced to two- 
thirds of original scale. Plant kept at rather too low a temperature. 

but by 8.25 A m. it occupied the position shown in the figure. 
The diagram differs greatly in appearance from most of those 
previously given; and this is due to the leaflet twisting and 
moving laterally as it approaches and comes into contact with 



394 MODIFIED CIRCUMNTJTATION. Chap. VII 

its fellow. The movement of another leaflet, when asleep, 
was traced between 6 p.m. and 10.35 p.m., and it clearly cir- 
eumnutated, for it continued for two hours to sink, then rose, 
and then sank still lower than it was at 6 p.m. It may be 
seen in the preceding figure (167) that the leaflet, when the 
plant was subjected to a rather low temperature in the house 
descended and ascended during the middle of the day in a 
somewhat zigzag line; but when kept in the hot-house from 
9 a.m. to 3 p.m. at a high but varying temperature (viz., between 
72° and 83° F.) a leaflet (with the petiole secured) circumnutated 
rapidly, for it made three large vertical ellipses in the course of 
the six hours. According to Brongniart, Marsilea pubescens sleeps 
like the present species. These plants are the sole cryptogamic 
ones known to sleep. 



Summary and Concluding Remarks on the Nijctitrojpic 
or Sleep-movements of Leaves. — That these movements 
are in some manner of high, importance to the plants 
which exhibit them, few will dispute who have ob- 
served how complex they sometimes are. Thus with 
Cassia, the leaflets which are horizontal during the 
day not only bencl at night vertically downwards with 
the terminal pair directed considerably backwards, but 
they also rotate on their own axes, so that their lower 
surfaces are turned outwards. The terminal leaflet 
of Melilotus likewise rotates, by which movement one 
of its lateral edges is directed upwards, and at the 
same time it moves either to the left or to the right, 
until its upper surface comes into contact with that ot 
the lateral leaflet on the same side, which has like- 
wise rotated on its own axis. With Arachis, all four 
leaflets form together during the night a single 
vertical packet ; and to effect this the two anterior 
leaflets have to move upwards and the two posterior 
ones forwards, besides all twisting on their own axes. 
In the genus Sida the leaves of some species move at 
night through an angle of 90° upwards, and of others 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 395 

through the same angle downwards. We have seen a 
similar difference in the nyctitropic movements of the 
cotyledons in the genus Oxalis. In Lupinus, again, 
the leaflets move either upwards or downwards ; and 
in some species, for instance L. luteus, those on one 
side of the star-shaped leaf move up, and those on -the 
opposite side move down ; the intermediate ones rota- 
ting on their axes ; and by these varied movements, the 
whole leaf forms at night a vertical star instead of a 
horizontal one, as during the day. Some leaves and 
leaflets, besides moving either upwards or downwards, 
become more or less folded at night, as in Bauhinia 
and in some species of Oxalis. The positions, indeed, 
which leaves occupy when asleep are almost infinitely 
diversified ; they may point either vertically upwards 
or downwards, or, in the case of leaflets, towards the 
apex or towards the base of the leaf, or in any inter- 
mediate position. They often rotate at least as much 
as 90° on their own axes. The leaves which arise 
from upright and from horizontal or much inclined 
branches on the same plant, move in some few cases 
in a different manner, as with Porlieria and Strephium. 
The whole appearance of many plants is wonderfully 
changed at night, as may be seen with Oxalis, and 
still more plainly with Mimosa. A bush of Acacia 
Farnesiana appears at night as if covered with little 
dangling bits of string instead of leaves. Excluding 
a few genera not seen by ourselves, about which we 
are in doubt, and excluding a few others the leaflets of 
which rotate at night, and do not rise or sink much, 
there are 37 genera in which the leaves or leaflets rise, 
often moving at the same time towards the apex or 
towards the base of the leaf, and 32 genera in which 
they sink at night. 

The nyctitropic movements of leaves, leaflets, and 



3D 6 MODIFIED CIRCUMNUTATION". Chap. VII 

petioles are effected in two different ways ; firstly, by 
alternately increased growth on their opposite sides, 
preceded by increased turgescence of the cells; and 
secondly by means of a pulvinus or aggregate of small 
cells, generally destitute of chlorophyll, which become 
alternately more turgescent on nearly opposite sides ; 
and this turgescence is not followed by growth except 
during the early age of the plant. A pulvinus seems 
to be formed (as formerly shown) by a group of cells 
ceasing to grow at a very early age, and therefore does 
not differ essentially from the surrounding tissues. 
The cotyledons of some species of Trifolium are pro- 
vided with a pulvinus, and others are destitute of one, 
and so it is with the leaves in the genus Sida. We 
see also in this same genus gradations in the state of 
the development of the pulvinus ; and in Nicotiana 
we have what may probably be considered as the 
commencing development of one. The nature of the 
movement is closely similar, whether a pulvinus is 
absent or present, as is evident from many of the 
diagrams given in this chapter. It deserves notice 
that when a pulvinus is present, the ascending and 
descending lines hardly ever coincide, so that ellipses 
are habitually described by the leaves thus provided, 
whether they are young or so old as to have quite 
ceased growing. This fact of ellipses being described, 
shows that the alternately increased turgescence of 
the cells does not occur on exactly opposite sides of the 
pulvinus, any more than the increased growth which 
causes the movements of leaves not furnished with 
pulvini. When a pulvinus is present, the nyctitropic 
movements are continued for a very much longer 
period than when such do not exist. This has been 
amply proved in the case of cotyledons, and Pfeffer 
has given observations to the same effect with respect 



Chap. VII SUMMARY ON SLEEP OF LEAVES. 39 1 

to leaves. We have seen that a leaf of Mimosa 
pudica continued to move in the ordinary manner, 
though somewhat more simply, until it withered and 
died. It may be added that some leaflets of Trifolium 
pratense were pinned open during 10 days, and on the 
first evening after being released they rose up and 
slept in the usual manner. Besides the long con- 
tinuance of the movements when effected by the aid 
of a pulvinus (and this appears to be the final cause 
of its development), a twisting movement at night, as 
PfefTer has remarked, is almost confined to leaves thus 
provided. 

It is a very general rule that the first true leaf, 
though it may differ somewhat in shape from the 
leaves on the mature plant, yet sleeps like them ; and 
this occurs quite independently of the fact whether or 
not the cotyledons themselves sleep, or whether they 
sleep in the same manner. But with Phaseolus Box- 
bur gliii the first unifoliate leaves rise at night almost 
sufficiently to be said to sleep, whilst the leaflets of 
the secondary trifoliate leaves sink vertically at night. 
On young plants of Sida rhomhuefolia, only a few 
inches in height, the leaves did not sleep, though on 
rather older plants they rose up vertically at night. 
On the other hand, the leaves on very young plants of 
Cytisus fragrans slept in a conspicuous manner, whilst 
on old and vigorous bushes kept in the greenhouse, 
the leaves did not exhibit any plain nyctitropic move- 
ment. In the genus Lotus the basal stipule-like 
leaflets rise up vertically at night, and are provided 
with pulvini. 

As already remarked, when leaves or leaflets change 
their position greatly at night and by complicated 
movements, it can hardly be doubted that these must 
be in some manner beneficial to the plant. If so, we 



398 MODIFIED CIRCUMNUTATION. Chap. V\l. 

must extend the same conclusion to a large number of 
sleeping plants; for the most complicated and tho 
simplest nyctitropic movements are connected together 
by the finest gradations. But owing to the causes spe- 
cified in the beginning of this chapter, it is impossible 
in some few cases to determine whether or not certain 
movements should be called nyctitropic. Generally, 
the position which the leaves occupy at night indi- 
cates with sufficient clearness, that the benefit thus 
derived, is the protection of their upper surfaces from 
radiation into the open sky, and in many cases the 
mutual protection of all the parts from cold by their 
being brought into close approximation. It should be 
remembered that it was proved in the last chapter, that 
leaves compelled to remain extended horizontally at 
night, suffered much more from radiation than those 
which were allowed to assume their normal vertical 
position. 

The fact of the leaves of several plants not sleeping 
unless they have been well illuminated during the 
day, made us for a time doubt whether the pro- 
tection of their upper surfaces from radiation was in 
all cases the final cause of their well-pronounced 
nyctitropic, movements. But we have no reason to 
suppose that the illumination from the open sky, 
during even the most clouded day, is insufficient for 
this purpose ; and we should bear in mind that leaves 
which are shaded from being seated low down on the 
plant, and which sometimes do not sleep, are likewise 
protected at night from full radiation. Nevertheless, 
we do not wish to deny that there may exist cases in 
which leaves change their position considerably at 
night, without their deriving any benefit from such 
movements. 

Although with sleeping plants the blades almost 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 399 

always assume at night a vertical, or nearly vertical 
position, it is a point of complete indifference whether 
the apex, or the base, or one of the lateral edges, is 
directed to the zenith. It is a rule of wide generality, 
than whenever there is any difference in the degree of 
exposure to radiation between the upper and the lower 
surfaces of leaves and leaflets, it is the upper which is 
the least exposed, as may be seen in Lotus, Cytisus, 
Trifolium, and other genera. In several species of 
Lupinus the leaflets do not, and apparently from 
their structure cannot, place themselves vertically at 
night, and consequently their upper surfaces, though 
highly inclined, are more exposed than the lower ; and 
here we have an exception to our rule. But in other 
species of this genus the leaflets succeed in placing 
themselves vertically ; this, however, is effected by a 
very unusual movement, namely, by the leaflets on 
the opposite sides of the same leaf moving in opposite 
directions. 

It is again a very common rule that when leaflets 
come into close contact with one another, they do so 
by their upper surfaces, which are thus best protected. 
In some cases this may be the direct result of their 
vising vertically ; but it is obviously for the pro- 
tection of the upper surfaces that the leaflets of 
Cassia rotate in so wonderful a manner whilst sinking 
downwards ; and that the terminal leaflet of Melilotus 
rotates and moves to one side until it meets' the lateral 
leaflet on the same side. When opposite leaves or 
leaflets sink vertically down without any twisting, 
their lower surfaces approach each other and some- 
times come into contact; but this is the direct and 
inevitable result of their position. With many species 
of Oxalis the lower surfaces of the adjoining leaflets 
are pressed together, and are thus better protected 



400 MODIFIED CIRCUMNUTATION. Chap. vu. 

than the upper surfaces ; but this depends merely ou 
each leaflet becoming folded at night so as to be able 
to sink vertically downwards. The torsion or rotation 
of leaves and leaflets, which occurs in so many cases, 
apparently always serves to bring their upper surfaces 
into close approximation with one another, or with 
other parts of the plant, for their mutual protection. 
We see this best in such cases as those of Arachis^ 
Mimosa albida, and Marsilea, in which all the leaflets 
form together at night a single vertical packet. If 
with Mimosa pudica the opposite leaflets had merely 
moved upwards, their upper surfaces would have come 
into contact and been well protected ; but as it is, 
they all successively move towards the apex of the 
leaf; and thus not only their upper surfaces are pro- 
tected, but the successive pairs become imbricated and 
mutually protect one another as well as the petioles. 
This imbrication of the leaflets of sleeping plants is a 
common phenomenon. 

The nycti tropic movement of the blade is gene- 
rally effected by the curvature of the uppermost part 
of the petiole, which has often been modified into a 
pulvinus ; or the whole petiole, when short, may be 
thus modified. But the blade itself sometimes curves 
or moves, of which fact Bauhinia offers a striking 
instance, as the two halves rise up and come intc 
close contact at night. Or the blade and the upper 
part of the petiole may both move. Moreover, the 
petiole as a whole commonly either rises or sinks at 
night. This movement is sometimes large : thus the 
petioles of Cassia pubescens stand only a little above 
the horizon during the day, and at night rise up 
almost, or quite, perpendicularly. The petioles of the 
younger leaves of Desmodium gyrans also rise up ver- 
tically at night. On the other hand, with Amphi- 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 401 

earpsea, the petioles of some leaves sank down as 
much as 57° at night; with Arachis they sank 39°, 
and then stood at right angles to the stem. Gene- 
rally, when the rising or sinking of several petioles on 
the same plant was measured, the amount differed 
greatly. This is largely determined by the age of the 
leaf: for instance, the petiole of a moderately old leaf 
of Desmodium gyrans rose only 46°, whilst the young 
ones rose up vertically ; that of a young leaf of Cassia 
floribunda rose 41°, whilst that of an older leaf rose 
only 12°. It is a more singular fact that the age of 
the plant sometimes influences greatly the amount of 
movement ; thus with some young seedlings of a Bau- 
hinia the petioles rose at night 30° and 34°, whereas 
those on these same plants, when grown to a height 
of 2 or 3 feet, hardly moved at all. The position of 
the leaves on the plant as determined by the light, 
seems also to influence the amount of movement 
of the petiole; for no other cause was apparent 
why the petioles of some leaves of Melilotus officinalis 
rose as much as 59°, and others only 7° and 9° at 
night. 

In the case of many plants, the petioles move at 
night in one direction and the leaflets in a directly 
opposite one. Thus, in three genera of Phaseolese the 
leaflets moved vertically downwards at night, and the 
petioles rose in two of them, whilst in the third they 
sank. Species in the same genus often differ widely 
in the movements of their petioles. Even on the same 
plant of Lupinus pubescens some of the petioles rose 3o° ? 
others only 6°, and others sank 4° at night. The 
leaflets of Cassia Barclayana moved so little at night 
that they could not be said to sleep, yet the petioles 
of some young leaves rose as much as 34°. These 
several facts apparently indicate that the movements 



102 MODIFIED CIKCUMNUTATION. Chap. VII 

of the petioles are not performed for any special pur- 
pose; though a conclusion of this kind is generally 
rash. When the leaflets sink vertically down at night 
and the petioles rise, as often occurs, it is certain that 
the upward movement of the latter does not aid the 
leaflets in placing themselves in their proper posi- 
tion at night, for they have to move through a 
greater angular space than would otherwise have been 
necessary. 

Notwithstanding what has just been said, it may be 
strongly suspected that in some cases the rising of 
the petioles, when considerable, does beneficially serve 
the plant by greatly reducing the surface exposed to 
radiation at night. If the reader will compare the 
two drawings (Fig. 155, p. 371) of Cassia pubescens, 
copied from photographs, he will see that the dia- 
meter of the plant at night is about one-third of 
what it is by day, and therefore the surface exposed 
to radiation is nearly nine times less. A similar 
conclusion may be deduced from the drawings (Fig. 
149, p. 358) of a branch awake and asleep of Des- 
modium gyrans. So it was in a very striking manner 
with young plants of Bauhinia, and with Oxalis 
Ortegesii. 

We are led to an analogous conclusion with respect 
to the movements of the secondary petioles of certain 
pinnate leaves. The pinnae of Mimosa pudica con- 
verge at night; and thus the imbricated and closed 
leaflets on each separate pinna are all brought close 
together into a single bundle, and mutually protect 
one another, with a somewhat smaller surface exposed 
to radiation. With Albizzia lophantha the pinnae close 
together in the same manner. Although the pinnae 
of Acacia Famesiana do not converge much, they 
sink downwards. Those of Neptunia oleracea likewise 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 403 

move downwards, as well as backwards, towards the 
base of the leaf, whilst the main petiole rises. With 
Schrankia, again, the pinnae are depressed at night. 
Now in these three latter cases, though the pinna) 
do not mutually protect one another at night, yet 
after having sunk down they expose, as does a 
dependent sleeping leaf, much less surface to the 
zenith and to radiation than if they had remained 
horizontal.. 

Any one who had never observed continuously a 
sleeping plant, would naturally suppose that the leaves 
moved only in the evening when going to sleep, and 
in the morning when awaking ; but he would be quite 
mistaken, for we have found no exception to the rule 
that leaves which sleep continue to move during the 
whole twenty -four hours ; they move, however, more 
quickly when going to sleep and when awaking than 
at other times. That they are not stationary during 
the day is shown by all the diagrams given, and by 
the many more which were traced. It is troublesome 
to observe the movements of leaves in the middle of 
the night, but this was done in a few cases ; and 
tracings were made during the early part of the night 
of the movements, in the case of Oxalis, Amphicarpa3a, 
two species of Erythrina, a Cassia, Passiflora, Euphorbia 
and Marsilea ; and the leaves after they had gone to 
s-leep, were found to be in constant movement. When, 
however, opposite leaflets come into close contact with 
one another or with the stem at night, they are, as we 
believe, mechanically prevented from moving, but this 
point was not sufficiently investigated. 

When the movements of sleeping leaves are traced 
during twenty -four hours, the ascending and descend- 
ing lines do not coincide, except occasionally and by 
accident for a shoi t space ; so that with many plants a 



i04 MODIFIED CIECUMNUTATION. Chap. VII 

single large ellipse is described during each twenty-foui 
hours. Such ellipses are generally narrow and ver- 
tically directed, for the amount of lateral movement is 
small. That there is some lateral movement is shown 
by the ascending and descending lines not coinciding, 
and occasionally, as with Besmodium gyrans and Thalia 
dealbata, it was strongly marked. In the case of Meli- 
lotus the ellipses described by the terminal leaflet 
during the day are laterally extended, instead of ver- 
tically, as is usual ; and this fact v evidently stands in 
relation with the terminal leaflet moving laterally 
when it goes to sleep. With the majority of sleeping 
plants the leaves oscillate more than once up and 
down in the twenty-four hoi?rs ; so that frequently two 
ellipses, one of moderate size, and one of very large size 
which includes the nocturnal movement, are described 
within the twenty-four hours. For instance, a leaf 
which stands vertically up during the night will sink 
in the morning, then rise considerably, again sink in 
the afternoon, and in the evening reascend and assume 
its vertical nocturnal position. It will thus describe, 
in the course of the twenty-four hours, two ellipses of 
unequal sizes. Other plants describe within the same 
time, three, four, or five ellipses. Occasionally the 
longer axes of the several ellipses extend in different 
directions, of which Acacia Farnesiana offered a good 
instance. The following cases will give an idea of the 
rate of movement : Oxalis acetosella completed two 
ellipses at the rate of 1 h. 25 m. for each ; Marsilea 
quadrifoliata, at the rate of 2 h.; Trifolium subterraneum, 
one in 3 h. 30 m. ; and Arachis liypogxa, in 4 h. 50 m. 
But the number of ellipses described within a given 
time depends largely on the state of the plant and 
on the conditions to which it is exposed. It often hap- 
pens that a single ellipse may be described during one 



Chap. VII. SUMMAKY ON SLEEP OF LEAVES. 405 

clay, and two on the next. Erythrina corallodendron 
made four ellipses on the first day of observation 
and only a single one on the third, apparently owing 
to having been kept not sufficiently illuminated and 
perhaps not warm enough. But there seems likewise 
to be an innate tendency in different species of the 
same genus to make a different number of ellipses in 
the twenty-four hours : the leaflets of Trifolium repens 
made only one ; those of T. resupinatum two, and those 
of T. subterraneum three in this time. Again, the 
leaflets of Oxalis Plumierii made a single ellipse ; those 
of 0. bupleurifolia, two ; those of 0. Valdiviana, two or 
three; and those of 0. aeetosella, at least five in the 
twenty-four hours. 

The line followed by the apex of a leaf or leaflet, 
whilst describing one or more ellipses during the day, 
is often zigzag, either throughout its whole course or 
only during the morning or evening : Eobinia offered 
an instance of zigzagging confined to the morning, 
and a similar movement in the evening is shown in 
the diagram (Fig. 126) given under Sida. The amount 
of the zigzag movement depends largely on the plant 
being placed under highly favourable conditions. But 
even under such favourable conditions, if the dots which 
mark the position of the apex are made at consider- 
able intervals of time, and the dots are then joined, 
the course pursued will still appear comparatively 
simple, although the number of the ellipses will be 
increased ; but if dots are made every two or three 
minutes and these are joined, the result often is that 
all the lines are strongly zigzag, many small loops, 
triangles, and other figures being also formed. This 
fact is shown in two parts of the diagram (Fig. 150) 
of the movements of Desmodium gyrans. Strepliium 
floribundum, observed under a high temperature, 



106 MODIFIED CIRCUMNUTATION. Chap. VII 

made several little triangles at the rate of 43 m. 
for each. Mimosa pudiea, similarly observed, de- 
scribed three little ellipses in 67 m. ; and the apex 
of a leaflet crossed -^q of an inch in a second, or 
0'12 inch in a minute. The leaflets of Averrhoa 
made a countless number of little oscillations when 
the temperature was high and the sun shining. The 
zigzag movement may in all cases be considered as 
an attempt to form small loops, which are drawn out 
by a prevailing movement in some one direction. The 
rapid gyrations of the little lateral leaflets of Des- 
modiuni belong to the same class of movements, 
somewhat exaggerated in rapidity and amplitude. 
The jerking movements, with a small advance and 
still smaller retreat, apparently not exactly in the 
same line, of the hypocotyl of the cabbage and of 
the leaves of Dionsea, as seen under the microscope, 
all probably come under this same head. We may 
suspect that we here see the energy which is freed 
during the incessant chemical changes in progress in 
the tissues, converted into motion. Finally, it should 
be noted that leaflets and probably some leaves, whilst 
describing their ellipses, often rotate slightly on their 
axes ; so that the plane of the leaf is directed first to 
one and then to another side. This was plainly seen 
to be the case with the large terminal leaflets of Des- 
modium, Erythrina and Arnphicarpsea, and is probably 
common to all leaflets provided with a pulvinus. 

With "espect to the periodicity of the movements of 
sleeping leaves, Pfeffer * has so clearly shown that 
this depends on the daily alternations of light and 
darkness, that nothing farther need be said on this 



* 'Die Perio'Iisclien Bewcgungen der Blattorgnne,' 1875, p. 30, ei 
•passim. 



Ohap. VII. SUMMARY ON SLEEP OF LEAVES. 407 

head. But we may recall the behaviour of Mimosa 
in the North, where the sun does not set, and the 
complete inversion of the daily movements by artificial 
light and darkness. It has also been shown by us, 
that although leaves subjected to darkness for a mode- 
rately long time continue to circumnutate, yet the 
periodicity of their movements is soon greatly dis- 
turbed, or quite annulled. The presence of light or 
its absence cannot be supposed to be the direct cause 
of the movements, for these are wonderfully diversified 
even with the leaflets of the same leaf, although all 
have of course been similarly exposed. The move- 
ments depend on innate causes, and are of an adaptive 
nature. The alternations of light and darkness 
merely give notice to the leaves that the period has 
arrived for them to move in a certain manner. We 
may infer from the fact of several plants (Tropreoluni, 
Lupinus, &c.) not sleeping unless they have been well 
illuminated during the day, that it is not the actual 
decrease of light in the evening, but the contrast 
between the amount at this hour and during the early 
part of the day, which excites the leaves to modify 
their ordinary mode of circumnutation. 

As the leaves of most plants assume their proper 
diurnal position in the morning, although light be 
excluded, and as the leaves of some plants continue to 
move in the normal manner in darkness during at 
least a whole day, we may conclude that the periodi- 
city of their movements is to a certain extent in- 
herited.* The strength of such inheritance differs 



* Pfeffer denies such inherit- " Nachwirkung," or Ihe after- 

p.nce; he attributes (' Die Period. effects of light and darkness. 

Bewegungen,' pp. 30-56) the But we are unable to follow his 

periodicity when prolonged for train of reasoning. There does 

a day or two in d..rkness, to not seem to be anymore reason foi 

27 



108 MODIFIED CIKCUMNUTATION. . Chap. VH 

much in different species, and seems never to be rigid ; 
for plants have been introduced from all parts of the 
world into our gardens and greenhouses ; and if their 
movements had been at all strictly fixed in relation to 
the alternations of day and night, they would have 
slept in this country at very different hours, which 
is not the case. Moreover, it has been observed that 
sleeping plants in their native homes change their 
times of sleep with the changing seasons. * 

We may now turn to the systematic list (p. 320), 
This contains the names of all the sleeping plants 
known to us, though the list undoubtedly is very 
imperfect. It may be premised that, as a general 
rule, all the species in the same genus sleep in 
nearly the same manner. But there are some ex- 
ceptions; in several large genera including many 
sleeping species (for instance, Oxalis), some do not 
sleep. One species of Melilotus sleeps like a Tri- 
folium, and therefore very differently from its con- 
geners ; so does one species of Cassia. In the genus 
Sida, the leaves either rise or fall at night ; and with 
Lupinus they sleep in three different methods. Re- 
turning to the list, the first point which strikes us, is 
that there are many more genera amongst the Legu- 
minosae (and in almost every one of the Leguminous 
tribes) than in all the other families put together; 
and we are tempted to connect this fact with the great 



attributing such movements to this effect must be produced on the 
cause than, for instance, the in- seeds by the long-continued culti- 
herited habit of winter and vation of the parent-plants under 
summer wheat to grow best at different climates, but no one pro- 
different seasons; fir this habit bably would call this the " Naeh« 
is lost after a few years, like the wirkung " of the climates, 
movements of leavi s in darkness * Pfefifer, ibid., p. 46. 
after a few days. No doubt some 






Crap. VII. SUMMARY ON SLEEP OF LEAVES. 40S 

mobility of the stems and leaves in this family, as 
shown by the large number of climbing species which 
it contains. Next to the Leguminosae come the Mal- 
vaceae, together with some closely allied families. But 
by far the most important point in the list, is that we 
meet with sleeping plants in 28 families, in all Ihe 
great divisions of the Phanerogamic series, and in one 
Cryptogam. Now, although it is probable that with 
the Leguminosse the tendency to sleep may have been 
inherited from one or a few progenitors and possibly 
so in the cohorts of the Malvales and Chenopodiales, 
yet it is manifest that the tendency must have been 
acquired by the several genera in the other families, 
quite independently of one another. Hence the ques- 
tion naturally arises, how has this been possible ? 
and the answer, we cannot doubt, is that leaves owe 
their nyctitropic movements to their habit of cir- 
cumnutating, — a habit common to all plants, and 
everywhere ready for any beneficial development or 
modification. 

It has been shown in the previous chapters that the 
leaves and cotyledons of all plants are continually 
moving up and down, generally to a slight but some- 
times to a considerable extent, and that they describe 
either one or several ellipses in the course of twenty- 
four hours ; they are also so far affected by the alter- 
nations of day and night that they generally, or 
at least often, move periodically to a small extent ; 
and here we have a basis for the development of the 
greater nyctitropic movements. That the movements 
of leaves and cotyledons which do not sleep come 
within the class of circumnutating movements cannot 
be doubted, for they are closely similar to those of 
hypocotyls, epicotyls, the stems of mature plants, and 
of various other organs. Now, if we take the simplest 



410 MODIFIED CIRCUMNUTATION. Chap. VII. 

case of a sleepiDg leaf, we see that it makes a single 
ellipse in the twenty-four hours, which resembles one 
described by a non-sleeping leaf in every respect, except 
that it is much larger. In both cases the course pursued 
is often zigzag. As all non-sleeping leaves are inces- 
santly circumnutating, we must conclude that a part 
at least of the upward and downward movement of one 
that sleeps, is due to ordinary circumnutation ; and it 
seems altogether gratuitous to rank the remainder of 
the movement under a wholly different head. With 
a multitude of climbing plants the ellipses which they 
describe have been greatly increased for another pur- 
pose, namely, catching hold of a support. With these 
climbing plants, the various circumnutating organs have 
been so far modified in relation to light that, differently 
from all ordinary plants, they do not bend towards it. 
With sleeping plants the rate and amplitude of the 
movements of the leaves have been so far modified in 
relation to light, that they move in a certain direction 
with the waning light of the evening and with the 
increasing light of the morning more rapidly, and to 
a greater extent, than at other hours 

But the leaves and cotyledons of many non-sleeping 
plants move in a much more complex manner than in 
the cases just alluded to, for they describe two, three, 
or more ellipses in the course of a day. Now, if a 
plant of this kind were converted into one that slept, 
one side of one of the several ellipses which each 
leaf daily describes, would have to be greatly increased 
in length in the evening, until the leaf stood ver- 
tically, when it would go on circumnutating about the 
same spot. On the following morning, the side of 
another ellipse would have to be similarly increased 
in length, so as to bring the leaf back again into its 
diurnal position, when it would again circumnutate 



CHAr. VII SUMMARY ON SLEEP OF LEAVES. 411 

until the evening. If the reader will lock, for in- 
stance, at the diagram (Fig. 142, p. 351), representing 
the nyctitropic movements of the terminal leaflet of 
Trifolium subterranewn, remembering that the curved 
broken lines at the top ought to be prolonged much 
higher up, he will see that the great rise in the evening 
and the great fall in the morning together form a 
large ellipse like one of those described during the 
daytime, differing only in size. Or, he may look at 
the diagram (Fig. 103, p. 236) of the 3£ ellipses 
described in the course of 6 h. 35 m. by a leaf of 
Lupinus speciosus, which is one of the species in this 
genus that does not sleep ; and he will see that by 
merely prolonging upwards the line which was already 
rising late in the evening, and bringing it down 
again next morning, the diagram would represent the 
movements of a sleeping plant. 

With those sleeping plants which describe several 
ellipses in the daytime, and which travel in a strongly 
zigzag line, often making in their course minute loops, 
triangles, &c, if as soon as one of the ellipses begins 
in the evening to be greatly increased in size, dots are 
made every 2 or 3 minutes and these are joined, the 
line then described is almost strictly rectilinear, in 
strong contrast with the lines made during the day- 
time. This was observed with Desmodium gyrans and 
Mimosa pudica. With this latter plant, moreover, the 
pinnae converge in the evening by a steady move- 
ment, whereas during the day they are continually 
converging and diverging to a slight extent. In all 
such cases it was scarcely possible to observe the 
difference in the movement during the day and even- 
ing, without being convinced that in the evening the 
plant saves the expenditure of force by not moving 
laterally, and that its whole energy is now expended 



112 MODIFIED CIRCUMNUTATION. Chap. VII 

in gaining quickly its proper nocturnal position by 
a direct course. In several other cases, for instance, 
when a leaf after describing during the day one or 
more fairly regular ellipses, zigzags much in the 
evening, it appears as if energy was being expended, 
so that the great evening rise or fall might coin- 
cide with the period of the day proper for this 
movement. 

The most complex of all the movements performed 
by sleeping plants, is that when leaves or leaflets, 
after describing in the daytime several vertically 
directed ellipses, rotate greatly on their axes in the 
evening, by which twisting movement they occupy 
a wholly different position at night to what they do 
during the day. For instance, the terminal leaflets 
of Cassia not only move vertically downwards in the 
evening, but twist round, so that their lower surfaces 
face outwards. Such movements are wholly, or almost 
wholly, confined to leaflets provided with a pulvinus. 
But this torsion is not a new kind of movement 
introduced solely for the purpose of sleep; for it 
has been shown that some leaflets whilst describing 
their ordinary ellipses during the daytime rotate 
slightly, causing their blades to face first to one side 
and then to another. Although we can see how the 
slight periodical movements of leaves in a vertical 
plane could be easily converted into the greater yet 
simple nycti tropic movements, we do not at present 
know by what graduated steps the more complex 
movements, effected by the torsion of the pulvini, 
have been acquired. A probable explanation could 
be given in each case only after a close investigation 
of the movements in all the allied forms. 

From the facts and considerations now advanced we 
may conclude that nyctitropism, or the sleep of leaves 



Chap. VII. MODIFIED CIRCUMNUTATION. 413 

and cotyledons, is merely a modification of their ordi- 
nary circumnutating movement, regulated in its period 
and amplitude by the alternations of light and dark- 
ness. The object gained is the protection of the upper 
surfaces of the leaves from radiation at night, often 
combined with the mutual protection of the several 
parts by their close approximation. In such cases as 
those of the leaflets of Cassia — of the terminal leaflets 
of Melilotus — of all the leaflets of Arachis, Marsilea, 
&c. — we have ordinary circumnutation modified to the 
extreme extent known to us in any of the several great 
classes of modified circumnutation. On this view of 
the origin of nyctitropism we can understand how it 
is that a few plants, widely distributed throughout the 
Vascular series, have been able to acquire the habit of 
placing the blades of their leaves vertically at night, 
that is, of sleeping, — a fact otherwise inexplicable. 

The leaves of some plants move during the day in 
a manner, which has improperly been called diurnal 
sleep ; for when the sun shines brightly on them, they 
direct their edges towards it. To such cases we shall 
recur in the following chapter on Heliotropism. It 
has been shown that the leaflets of one form of 
Porlieria hygrometrica keep closed during the day, as 
long as the plant is scantily supplied with water, in 
the same manner as when asleep ; and this apparently 
serves to check evaporation. There is only one other 
analogous case known to us, namely, that of certain 
Gramineae, which fold inwards the sides of their narrow 
leaves, when these are exposed to the sun and to a 
dry atmosphere, as described by Duval- Jouve.* We 
have also observed the same phenomenon in Elymus 
arenareus. 



* » Annal. des Sc. Nat. (Bpt.),' 1875. torn. i. pp. 32c -329. 



114 STKUCTURE OF Chap. VII 

There is another movement, which since the time 
of Linnaeus has generally been called sleep, namely^ 
that of the petals of the many flowers which close at 
night. These mo Fements have been ably investigated 
by PfefTer, who has shown (as was first observed by 
Hofmeister) that they are caused or regulated more 
by temperature than by the alternations of light and 
darkness. Although they cannot fail to protect the 
organs of reproduction from radiation at night, this 
does not seem to be their chief function, but rather 
the protection of the organs from cold winds, and 
especially from rain, during the day. The latter 
seems probable, as Kerner * has shown that a widely 
different kind of movement, namely, the bending down 
of the upper part of the peduncle, serves in many 
cases the same end. The closure of the flowers will 
also exclude nocturnal insects which may be ill-adapted 
for their fertilisation, and the well-adapted kinds at 
periods when the temperature is not favourable for 
fertilisation. Whether these movements of the petals 
consist, as is probable, of modified circumnutation we 
do not know. 

Emhnjology of Leaves. — A few facts have been in- 
cidentally given in this chapter on what may be called 
the embryology of leaves. With most plants the 
first leaf which is developed after the cotyledons, 
resembles closely the leaves produced by the mature 
plant, but this is not always the case. The first 
leaves produced by some species of Drosera, for instance 
by D. Capensis, differ widely in shape from those 
borne by the mature plant, and resemble closely the 

eaves of D. rotundifolia, as was shown to us by Prof. 

Williamson of Manchester. The first true leaf of 



Die Schutzmittt 1 des Pollens,' 1873, pp. 30-39. 



Chap. VII. FIRST-FORMED LEAVES. 415 

the gorse, or Ulex, is not narrow and spinose like the 
older leaves. On the other hand, with many Legumi- 
nous plants, for instance, Cassia, Acacia lophantka, &c, 
the first leaf has essentially the same character as the 
older leaves, excepting that it bears fewer leaflets. In 
Trifolium the first leaf generally bears only a single 
leaflet instead of three, and this differs somewhat in 
shape from the corresponding leaflet on the older leaves. 
Now, with Trifolium Pannonicum the first true leaf on 
some seedlings was unifoliate, and on others completely 
trifoliate ; and between these two extreme states there 
were all sorts of gradations, some seedlings bearing 
a single leaflet more or less deeply notched on one 
or both sides, and some bearing a single additional 
and perfect lateral leaflet. Here, then, we have the 
rare opportunity of seeing a structure proper to a more 
advanced age, in the act of gradually eucroaching on 
and replacing an earlier or embryological condition. 

The genus Melilotus is closely allied to Trifolium, and 
the first leaf bears only a single leaflet, which at night 
rotates on its axis so as to present one lateral edge to 
the zenith. Hence it sleeps like the terminal leaflet 
of a mature plant, as was observed in 15 species, and 
wholly unlike the corresponding leaflet of Trifolium, 
which simply bends upwards. It is therefore a curious 
fact that in one of these 15 species, viz., M. Taurica (and 
in a lesser degree in two others), leaves arising from 
young shoots, produced on plants which had been cut 
down and kept in pots during the winter in the green- 
house, slept like the leaves of a Trifolium, whilst the 
leaves on the fully-grown branches on these same 
plants afterwards slept normally like tl ose of a Meli- 
lotus. If young shoots rising from the ground may 
be considered as new individuals, partaking to a certain 
extent of the nature of seedlings, then the peculiar 
manner in which their leaves slept may be considered 



4:16 STRUCTURE OF Chap. VII. 

as an erubryological habit, probably the result of Meli- 
lotus being descended from some form which slept like 
a Trifolimn. This view is partially supported by th6 
leaves on old and young branches of another species, 
M. Messanensis (not included in the above 15 species); 
always sleeping like those of a Trifolium. 

The first true leaf of Mimosa alb i da consists of a 
simple petiole, often bearing three pairs of leaflets, all 
of which are of nearly equal size and of the same 
shape : the second leaf differs widely from the first, 
and resembles that on a mature plant (see Fig. 159, 
p. 379), for it consists of two pinnae, each of which 
bears two pairs of leaflets, of which the inner basal 
one is very small. But at the base of each pinna 
there is a pair of minute points, evidently rudiments 
of leaflets, for they are of unequal sizes, like the two 
succeeding leaflets. These rudiments are in one sense 
embryological, for they exist only during the youth of 
the leaf, falling off and disappearing as soon as it is 
fully grown. 

With Desmodium gyrans the two lateral leaflets are 
very much smaller than the corresponding leaflets in 
most of the species in this large genus ; they vary 
also in position and size ; one or both are sometimes 
absent ; and they do not sleep like the fully-developed 
leaflets. They may therefore be considered as almost 
rudimentary ; and in accordance with the general prin- 
ciples of embryology, they ought to be more constantly 
and fully developed on very young than on old plants. 
But this is not the case, for they were quite absent 
on some young seedlings, and did not appear until 
from 10 to 20 leaves had been formed. This fact 
leads to the suspicion that D. gyrans is descended 
through a unifoliate form (of which some exist) from 
a trifoliate species ; and that the little lateral leaflets 
reappear through reversion. However this may be, 



Chap. VII. FIKST-FORMED LEAVES. 417 

the interesting fact of the pulvini or organs of move- 
ment of these little leaflets, not having been reduced 
nearly so much as their blades — taking the large 
terminal leaflet as the standard of comparison — gives 
us probably the proximate cause of their extraoidiuaiy 
pov or of gyration. 



4J8 MODIFIED CIRCUMNUTATION. Chap. VI1L 



CHAPTEE VIII. 

Modified Ciroumnutation : Movements excited by Light. 

Dintinction between heliotropism and the effects of light on the perio- 
dicity of the movements of leaves — Heliotropic movements of Beta, 
Solanum, Zea, and Avena — Heliotropic movements towards an 
obscure lkjht in Apios, Brassica, Phalaris, Tropseolum, and Cassia 
— Apheliotropic movements of tendrils of Bignonia — Of flower- 
peduncles of Cyclamen — Burying of the pods — Heliotropism 
and apheliotropism modified forms of circumnutation — Steps by 
which one movement is converted into the other — Transversal- 
heliotropismus or diaheliotropism, influenced by epinasty, the 
weight of the part and apogeotropism — Apogeotropism overcome 
during the middle of the day by diaheliotropism — Effects of t lie 
weight of the blades of cotyledons — So-called diurnal sleep — Chloro- 
phyll injured by intense light — Movements to avoid interne light. 

Sachs first clearly pointed out the important dif- 
ference between the action of light in modifying the 
periodic movements of leaves, and in causing them to 
bend towards its source.* The latter, or heliotropic 
movements are determined by the direction of the light, 
whilst periodic movements are affected by changes in 
its intensity and not by its direction. The periodicity 
of the circumnutating movement often continues for 
some time in darkness, as we have seen in the last 
chapter ; whilst heliotropic bending ceases very quickly 
when the light fails. Nevertheless, plants which have 
ceased through long-continued darkness to move pe- 
riodically, if re-exposed to the light are still, according 
to Sachs, heliotropic. 

Apheliotropism, or, as usually designated, negative 

* 'Physiologic Veg' (French Translation), 1868, pp.42, 517, &c. 



Chap. VIII. MOVEMENTS EXCITED BY LIGHT. 419 

heliotropism, implies that a plant, when unequally 
illuminated on the two sides, bends from the light, 
instead of, as in the last sub-class of cases, towards it ; 
but apheliotropism is comparatively rare, at least in a 
w^U-marked degree. There is a third and large sub- 
class of cases, namely, those of " Transversal- Helio- 
tropismus " of Frank, which we will here call diahelio- 
tropism. Parts of plants, under this influence, place 
themselves more or less transversely to the direction 
whence the light proceeds, and are thus fully illumi- 
nated. There is a fourth sub-class, as far as the final 
cause of the movement is concerned ; for the leaves of 
some plants when exposed to an intense and injurious 
amount of light direct themselves, by rising or sinking 
or twisting, so as to be less intensely illuminated. 
Such movements have sometimes been called diurnal 
sleep. If thought advisable, they might be called 
paraheliotropic, and this* term would correspond with 
our other terms. 

It will be shown in the present chapter that all the 
movements included in these four sub-classes, con- 
sist of modified circumnutation. We do not pretend to 
say that if a part of a plant, whilst still growing, did not 
circumnutate — though such a supposition is most im- 
probable j — it could not bend towards the light ; but, as 
a matter of fact, heliotropism seems always to consist 
of modified circumnutation. Any kind of movement 
in relation to light will obviously be much facilitated 
by each part circumnutating or bending successively 
in all directions, so that an already existing movement 
has only to be increased in some one direction, and to 
be lessened or stopped in the other directions, in order 
that it should become heliotropic, apheliotropic, &c, 
as the case may be. In the next chapter some obser- 
vations on the sensitiveness of plants to light, their 



420 



MODIFIED CIKCUMNUTATION. 



Chap VIU 



rate of bending towards it, and the accuracy with 
which they point towards its source, &c, will be 
given. Afterwards it will be shown — and this seems 
to us a point of much interest — that sensitiveness to 
light is sometimes confined to a small part of the 
plant; and that this part when stimulated by light, 
transmits an influence to distant parts, exciting them 
to bend. 

Heliotropism. — When a plant which is strongly 
heliotropic (and species differ much in this respect) 
is exposed to a bright lateral light, it bends quickly 
towards it, and the course pursued by the stem is 
quite or nearly straight. But if the light is much 
dimmed, or occasionally interrupted, or admitted in 
only a slightly oblique direction, 
the course pursued is more or less 
zigzag ; and as we have seen and 
shall again see, such zigzag move- 
ment results from the elongation or 
drawing out of the ellipses, loops, 
&c, which the plant would have de- 
scribed, if it had been illuminated 
from above. On several occasions 
we were much struck with this fact, 
whilst observing the circumnuta- 
tion of highly sensitive seedlings, 
which were unintentionally illu- 
minated rather obliquely, or only 
at successive intervals of time. 



Fig. 168. 




Beta vulga -''s : circumnu- 
tation of hypocotyl, de- 
flected by the light 
being slightly lateral, 
traced on a horizontal 
glass from 8.30 A.M. to 
5.30 p.m. Direction of the 
lighted taper by which 
it was illuminated, 
shown by a line joining 
the first and penultimate 
dots. Figure reduced to 
one-third of the original 
scale. 



For instance, two young seedlings of 
Beta vulgaris were placed in the middle 
of a room with north-east windows, and 
were kept covered up, except during 
each observation which lasted for only a minute or two ; but the 
result was that their hypocotyls bowed themselves to the side, 
whence some light occasionally entered, in lines which were 



Chap. VIII. 



HELIOTROPISM. 



421 



only slightly zigzag. Although not a single ellipse was eTen 
approximately formed, we inferred from the zigzag lines — and, 
as it proved, correctly — that their hypocotyls were circumnuta- 
ting, for on the following day these same seedlings were placed 
in a completely darkened room, and were observed each time by 
the aid of a small wax taper held almost 
directly above them, and their movements 
were traced on a horizontal glass above ; 
and now their hypocotyls clearly circum- 
nutated (Fig. 168, and Fig. 39, formerly 
given, p. 52); yet they moved a short 
distance towards the side where the taper 
was held up. If we look at these diagrams, 
and suppose that the taper had been held 
more on one side, and that the hypocotyls, 
still circumnutating, had bent themselves 
within the same time much more towards 
the light, long zigzag lines would ob- 
viously have been the result. 

Again, two seedlings of Solanum Jy co- 
pers i cum were illuminated from above, 
but accidentally a little more light entered 
on one than on any other side, and their 
hypocotyls became slightly bowed towards 
the brighter side ; they moved in a zigzag- 
line and described in their course two little 
triangles, as seen in Fig. 37 (p. 50), and 
in another tracing not given. The sheath- 
like cotyledons of Zea mays behaved, under 
nearly similar circumstances, in a nearly 8°a.m ; 

similar manner, as described in our first ^nasativa: heliotrope 

movement and circum- 




nutation of sheath-like 
cotyledon (1J inch in 
height) traced on hori- 
zontal glass from 8 A.M. 
to 10.25 P.M. Oct. Kith. 



chapter (p. 64), for they bowed themselves 

during the whole day towards one side, 

making, however, in their course some 

conspicuous flexures. Before we knew 

how greatly ordinary circumnutation was 

modified by a lateral light, some seedling oats, with rather old 

and therefore not highly sensitive cotyledons, were placed in 

front of a north-east window, towards which they bent all day in 

a strongly zigzag course. On the following day they continued 

to bend in the same direction (Fig. 169), but zigzagged much 

less. The sky, however, became between 12.40 and 2.35 P.Bt. 



122 MODIFIED CIRCUMNUTATION. Chap. VIII. 

overcast with extraordinarily dark thunder-clouds, and it was 
interesting to note how plainly the cotyledons circumnutated 
during tins interval. 

The foregoing observations are of some 

*ig. 170. value, from having then made when we were 

1° not attending to heliotropism ; and they led 

; % us to experiment on several kinds of seed- 

*£ lings, by exposing them to a dim lateral light, 

| S so as to observe the gradations between 

\ 2 ordinary circumnutation and heliotropism. 

j.Sf- Seedlings in pots were placed in front of, 

s and about a yard from, a north-east window ; 

\ ^ on each side and over the pots black boards 

: - 1 were placed ; in the rear the pots were open 

3 U, to the diffused light of the room, which 

: rr - had a second north-east and a north-west 

\ q window. By hanging up one or more blinds 

- o before the window where the seedlings stood, 

; J; it was easy to dim the light, so that very 

; o little more entered on this side than on the 

3 j opposite one, which received the diffused 

\ < light of the room. Late in the evening the 

J'S blinds were successively removed, and as the 

I -g g plants had been subjected during the day to 

g. 2 a very obscure light, they continued to bend 

c m towards the window later in the evening than 

g «S would otherwise have occurred. Most of the 

£-3 seedlings were selected because they were 

g g known to be highly sensitive to light, and 

% •£ some because they were but little sensitive, 

2 ^ . or had become so from having grown old. 

•gLfl^ The movements were traced in the usual 

S^Z manner on a horizontal glass cover; a fine 

~ I B glass filament with little triangles of paper 

A *^*S having been cemented in an upright position 

§£"3 to the hypocotyls. Whenever the stem or 

"S ^ "£ hypocotyl became much bowed towards the 

§ "* '% light, the latter part of its course had to 

^-f S be traced on a vertical glass, parallel to the 

•S "* window, and at right angles to the horizontal 

^ glass cover. 

Apios graveolens. — The hypocotyl bends in a few hours rectan- 



i 



Chap VIII. HELIOTROPISM. 423 

gularly towards a bright lateral light. In order to ascertain 
how straight a course it would pursue when fairly well illumi- 
nated on one side, seedlings were first placed before a south-west 
window on a cloudy and rainy morning ; and the movement of 
two hypocotyls were traced for 3 h., during which time they 
became greatly bowed towards the light. One of these tracings 
is given on p. 422 (Fig. 170), and the course may be seen to be 
almost straight. But the amount of light on this occasion was 
superfluous, for two seedlings were placed before a north-east 
window, protected by an ordinary linen and two muslin blinds, 
yet their hypocotyls moved towards this rather dim light in 
only slightly zigzag lines; but after 4p.m., as the light waned, 
the lines became distinctly zigzag. One of these seedlings, 
moreover, described in the afternoon an ellipse of considerable 
size, with its longer axis directed towards the window. 

We now determined that the light should be made dim 
enough, so we began by exposing several seedlings before a 
north-east window, protected by one linen blind, three muslin 
blinds, and a towel. But so little light entered that a pencil 
cast no perceptible shadow on a white card, and the hypocotyls 
did not bend at all towards the window. During this time, 
from 8.15 to 10.50 a.m., the hypocotyls zigzagged or circum- 
nutated near the same spot, as may be seen at A, in Fig. 171. 
The towel, therefore, was removed at 10.50 a.m., and replaced 
by two muslin blinds, and now the light passed through 
one ordinary linen and four muslin blinds. When a pencil 
was held upright on a card close to the seedlings, it cast a 
shadow (pointing from the window; which could only just 
be distinguished. Yet this very slight excess of light on 
one side sufficed to cause the hypocotyls of all the seedlings 
immediately to begin bending in zigzag lines towards the 
window. The course of one is shown at A (Fig. 171): after 
moving towards the window from 10.50 a.m. to 12.48 p.m. it 
bent from the wiudow, and then returned in a nearly parallel 
line; that is, it almost completed between 12.48 and 2 p.m. 
a narrow ellipse. Late in the evening, as the light waned, 
the hypocotyl ceased to bend towards the window, and circum- 
nutated on a small scale round the same spot ; during the night 
it moved considerably backwards, that is, became more upright, 
through the action of apogeotropism. At B, we have a tracing 
of the movements of another seedling from the hour (10.50 a.m.) 
when the towel was removed ; and it is in all essential respects 



424 



MODIFIED CIECUMNUTATION. 



Chap. V1U 



similar to the previous one. In these two cases there could be 
no doubt that the ordinary circurnnutating movement of the 
hypocotyl was modified and rendered heliotropic. 

Fig. 171. 




12.48,'* 



10°. 50 a* 




815 f a.m* 



10>~50 a.m. 



Apios grareolens : heliotropic movement and circumnutation of the hypo- 
cotyls of two seedlings towards a dim lateral light, traced on a horizontal 
glass during the day. The broken lines show their return nocturnal 
courses. Height of hypocotyl of A -5, and of B "55 inch. Figure reduceJ 
to one-half of original scale. 

Brassica oleracea. — The 'hypocotyl of the cabbage, when not 
disturbed by a lateral light, circumnutates in a complicated 



Chap. VIII. 



HELIOTROriSM. 



42^ 



manner over nearly the same space, and a figure formerly given 
is here reproduced (Fig. 172). If the hypocotyl is exposed to 
a moderately strong lateral light it moves quickly towards this 
side, travelling in a straight, or nearly straight, line. But when 
the lateral light is very dim its course is extremely tortuous, and 
evidently consists of modified circumnutation. Seedlings were 
placed before a north-east window, protected by a linen and 
muslin blind and by a towel. The sky was cloudy, and when- 
ever the clouds grew a little lighter an additional muslin blind 
was temporarily suspended. The light from the window was 

Fig. 172. 




Brassica olcracea ordinary circumnutating increment of the hj-pocoty! of 
a seedling plant. 



thus so much obscured that, judging by the unassisted eye, the 
seedlings appeared to receive more light from the interior 
of the room than from the window; but this was not really 
the case, as was shown by a very faint shadow cast by a pencil 
on a card. Nevertheless, this extremely small excess of light 
on one side caused the hypocotyls, which in the morning had 
stood upright, to bend at right angles towards the window, 
so that in the evening (after 4.23 p.m.) their course had to be 
traced on a vertical glass parallel to the window. It should be 
stated that at 3.30 p.m., by which time the sky had become 
darker, the towel was removed and replaced by an additional 
muslin blind, which itself was removed at 4 p.m., the other two 



126 



MODIFIED CIRCUMNUTATION. 



Chap. VilL 



blinds being left suspended. In Fig. 173 the course pursued, 
between 8.9 a.m. and 7.10 p.m., by one cf the hypocotyls thiia 




>8°25' 

Brasska oleracea : heliotropic movement and circumnutation of a hypocotyl 
towards a very dim lateral light, traced during 11 hours, on a horizontal 
glass in the morning, and on a vertical glass in the evening. Figure 
reduced to one-third of the original scale. 

exposed is shown. It may be observed that during the first 
16 m. tl:e hypocotyl moved obliquely from the light, and this, 



Chap. VIII. 



HELIOTEOPISM. 



127 



no doubt, was due to its then circumnutating in this direction 
Similar cases were repeatedly observed, and a dim light rarely 
or never produced any effect until from a quarter to three- 
quarters of an hour had elapsed. After 5.15 p.m., by which 
time the light had become 
obscure, the hypocotyl 
began to circumnutate 
about the same spot. The 
contrast between the two 
figures (172 and 173) 
would have been more 
striking, if they had been 
originally drawn on the 
same scale, and had been 
equally reduced. But the 
movements shown in Fig. 
1 72 were at first more mag- 
nified, and have been re- 
duced to only one-half of 
the original scale; whereas 
those in Tig. 173 were at 
first less magnified, and 
have been reduced to a 
one-third scale. A tracing 
made at the same time 
with the last of the 
movements of a second 
hypocotyl, presented a 
closely analogous appear- 
ance ; but it did not bend 
quite so much towards the 
light, and it circumnu- 
tated rather more plainly. 
Phalaris Canariensis. — 
The sheath-like cotyledons 
of this monocotyledonous 
plant were selected for 
trial, because they are very sensitive to light and circumnutate 
well, as formerly shown (see Fig. 49, p. 63). Although we felt 
no doubt about the result, some seedlings were first placed 
before a south-west window on a moderately bright morning, and 
the movements of one were traced. As is so common, it moved 




8?lS 1 a.m.Sepl6. t . h 



Phalaris Canariensis: heliotropic movement 
and circumnutation of a rather old coty- 
ledon, towards a dull lateral light, traced 
on a horizontal glass from 8.15 A.M. Sept. 
16th to 7.45 A.M. 17th. Figure reduced 
to one-third of original scale. 



128 MODIFIED CIKCUMNUTATION. Chap. VIII, 

for the first 45 m. in a zigzag line ; it then felt the fnll influence 
of the light, and travelled towards it for the next 2 h. 30 m. in an 
almost straight line. The tracing has not been given, as it was 
almost identical with that of Apios under similar circum- 
stances (Fig. 170). By noon it had bowed itself to its full 
extent ; it then circumnutated about the same spot and described 
two ellipses ; by 5 p.m. it had retreated considerably from the 
light, through the action of apogeotropism. After some pre- 
liminary trials for ascertaining the right degree of obscurity, 
some seedlings were placed (Sept. 16th) before a north-east 
window, and light was admitted through an ordinary linen 
and three muslin blinds. A pencil held close by the pot now 
cast a very faint shadow on a white card, pointing from the 
window. In the evening, at 4.30, and again at 6 p.m., some of 
the blinds were removed. In Fig. 174 we see the course pursued 
under these circumstances by a rathsr old and not very sensitive 
cotyledon, 1*9 inch in height, which became much bowed, 
but was never rectangularly bent towards the light. From 
11 a.m., when the sky became rather duller, until 6.30 p.m., the 
zigzagging was conspicuous, and evidently consisted of drawn- 
out ellipses. After 6.30 p.m. and during the night, it retreated 
in a crooked line from the window. Another and younger seed- 
ling moved during the same time much more quickly and to a 
much greater distance, in an only slightly zigzag line towards 
the light ; by 11 a.m. it was bent almost rectangularly in this 
direction, and now circumnutated about the same place. 

Tropceolum majus. — Some very young see llings, bearing only 
two leaves, and therefore not as yet arrived at the climbing 
stage of growth, were first tried before a north-east window 
without any blind. The epicotyls bowed themselves towards 
the light so rapidly that in little more than 3 h. their tips 
pointed rectangularly towards it. The lines traced were either 
nearly straight or slightly zigzag ; and in this latter case we 
see that a trace of circumnutation was retained even under the 
influence of a moderately bright light. Twice whilst these 
epicotyls were bending towards the window, dots were made 
every 5 or 6 minutes, in order to detect any trace of lateral 
movement, but there was hardly any ; and the lines formed by 
their junction were nearly straight, or only very slightly zigzag, 
as in the other pirts of the igures. After the epicotyls had 
bowed themselves to the full extent towards the light, ellipses 
of considerable size were described in the usual manner. 



Chap. VIII. 



HELIOTKOPISM. 



429 



After haying seen how the epicotyls moTed towards a mode 
rately bright light, seedlings were placed at 7.48 a.m. (Sept. 7th) 
before a north-east window, covered by a towel, and shortly 
afterwards by an ordinary linen blind, but the epicotyls still 
moved towards the window. At 9.13 a.m. two additional muslin 
blinds were suspended, so that the seedlings received very little 
more light from the window than from the interior of the room 
The sky varied in brightness, and the seedlings occasionally 



Fig. 175. 



6M\ 



i0°40'jo.m. 




JG>dO'p.m, 



7°48'a.trf 

Tropceolum majus : heliotropic movement and circumnutation of the epicotyl 
of a young seedling towards a dull lateral light, traced on a horizontal 
glass from 7.48 A.M. to 10.40 p.m. Figure reduced to one-half of the 
original scale. 



received for a short time less light from the window than from 
the opposite side (as ascertained by the shadow cast), and then 
one of the blinds was temporarily removed. In the evening 
tho blinds were taken away, one by one. The course pursued 
by an epicotyl under these circumstances is shown in Fig. 175. 
During the whole day, until 6.45 p.m., it plainly bowed itself 
towards the light ; and the tip moved over a considerable space. 
After 6.45 p.m. it moved backwards, or from the window, till 



430 



MODIFl ED CIKCUMNUTATION. 



Chap. VIII 



Fig. 176. 




10.40 p.m., when the last dot was made. Here, then, we have 
a distinct heliotropic movement, effected by means of six 
elongated figures (which if dots had been made every few 
minutes would have been more or less elliptic) directed towards 

the light, with the apex of each suc- 
cessive ellipse nearer to the window 
than the previous one. Now, if the 
light had been only a little brighter, 
the epicotyl would have bowed itself 
more to the light, as we may safely 
conclude from the previous trials ; 
there would also have been less 
lateral movement, and the ellipses or 
other figures would have been drawn 
out into a strongly marked zigzag 
line, with probably one or two small 
loops still formed. If the light had 
been much brighter, we should have 
had a slightly zigzag line, or one 
quite straight, for there would have 
been more movement in the direc- 
tion of the light, and much less from 
side to side. 

Sachs states that the older inter- 
nodes of this Tropseolum are aphe- 
liotopic; we therefore placed a 
plant, 111 inches high, in a box, 
blackened within, but open on one 
side in front of a north-east window 
without any blind. A filament was 
fixed to the third internode from 
the summit on one plant, and to 
the fourth internode of another. 
These internodes were either not 
old enough, or the light was not suf- 
ficiently bright, to induce aphelio- 
tropism, for both plants ben^ slowly towards, instead of from 
the window during four days. The course, during two da\ s of 
the first-mentioned internode, is given in Fig. 176 ; and we see 
that it either circumnutated on a small scale, or travelled in a 
zigzag line towards the light. We have thought this case of 
feeble heliotropism in one of the older internodes of a plant, 




Tropceolum majus : heliotropic 
movement and circumnuta- 
tion of an old internode to- 
wards a lateral light, traced 
on a horizontal glass from 8 
A.M. Nov. 2nd to 10.20 A.M. 
Nov. 4th. Broken lines show 
the nocturnal course. 



Chap. VI EI. 



HELIOTROPISM. 



431 



which, whilst young, is so extremely sensitive to light, worth 
giving. 

Cassia tora. — The cotyledons of this plant are extremely 
sensitive to light, whilst the 



mnufjpmT 



Ch 



hypocotyls are much less 
sensitive than those of most 
other seedlings, as we had 
often observed with surprise. 
It seemed therefore worth 
while to trace their move- 
ments. They were exposed 
to a lateral light before a 
north-east window, which 
was at first covered merely 
by a muslin blind, but as 
the sky grew brighter about 
11 a.m., an additional linen 
olind was suspended. After 
4 p.m. one blind and then the 
other was removed. The 
seedlings were protected on 
each side and above, but were 
open to the diffused light 
of the room in the rear. Up- 
right filaments were fixed to 
the hypocotyls of two seed- 
lings, which stood vertically 
in the morning. The accom- 
panying figure (Fig. 177) 
shows the course pursued by 
one of them during two days ; 
but it should be particularly 
noticed that during the 
second day the seedlings were 
kept in darkness, and they 
then circumnutated round 
nearly the same small space. 
On the first day (Oct. 7th) 
the hypocotyl moved from 
8 a.m. to 12.23 p.m., toward 
the light in a zigzag line, then turned abruptly to the left 
and afterwards described a small ellipse. Another irregular 




Qy.mJ 



tl 



8 a.m.'/ -.4 

Cassia tora: heliotropic movement and 
circumnntation of a hypocotyl (1| 
inch in height) traced on a horizontal 
glass from 8 A.M. to 10.10 P.M. Oct. 
7th. Also its circumnntation in 
darkness from 7 A.M. Oct. 8th to 7.45 
A.M. Oct. 9th. 



132 



MODIFIED CIRCUMNUTATION. Chap. VIII 



ellipse was completed between 3 p.m. and abont 5.30 p.m., 
the hypocotyl still bending towards the light. The hypocotyl 



Fig. 178. 





\ 




Bignonia capreolata : aphe- 
liotropic movement of a 
tendril, traced on a hori- 
zontal glass from 6.45 
A.M. July 19th to 10 a.m. 
20th. Movements as 
originally traced, little 
magnified, here reduced 
to two-thirds of the 
original scale. 

stationary ; they 



was straight and upright in the morn- 
ing, but by 6 p.m. its upper half was 
bowed towards the light, so that the 
chord of the arc thus formed stood at 
an angle of 20° with the perpendicular. 
After 6 p.m. its course was reversed 
through the action of apogeotropism, 
and it continued to bend from the 
window during the night, as shown by 
the broken line, On the next day it 
was kept in the dark (excepting when 
each observation was made by the aid 
of a taper), and the course followed 
from 7 a,m on the 8th to 7.45 a.m. on 
the 9th is here likewise shown. The 
difference between the two parts of the 
figure (177), namely, that described 
during the daytime on the 7th, when 
exposed to a rather dim lateral light, 
and that on the 8th in darkness, is 
striking. The difference consists in the 
lines during the first day having been 
drawn out in the direction of the light. 
The movements of the other seedling, 
traced under the same circumstances, 
were closely similar. 

Apheliotropism. — We succeeded in 
observing only two cases of aphelio- 
tropism, for these are somewhat rare ; 
and the movements are generally so 
slow that they would have been very 
troublesome to trace. 

Bignonia capreolata. — No organ of 
any plant, as far as we have seen, bends 
away so quickly from the light as do 
the tendrils of this Bignonia. They 
are also remarkable from circum- 
nutating much less regularly than 
most other tendrils, often remaining 
depend on apheliotropism for corning into 






(jiiap. VIII. AFHELIOTROPISM. 438 

contact with the trunks of trees.* The stem of a young plant 
was tied to a stick at the base of a pair of fine tendrils, which 
projected almost vertically upwards; and it was placed in 
front of a north-east window, being protected on all other sides 
from the light. The first dot was made at 6.45 a.m., and by 
7.35 a.m. both tendrils felt the full influence of the light, for 
they moved straight away from it until 9.20 a.m., when they 
circumnutated for a time, still moving, but only a little, from 
the light (see Fig. 178 of the left-hand tendril). After 3 p.m. 
they again moved rapidly away from the light in zigzag lines. 
By a late hour in the evening both had moved so far, that 
they pointed in a direct line from the light. During the night 
they returned a little in a nearly opposite direction. On the 
following morning they again moved from the light and con- 
verged, so that by the evening they had become interlocked, 
still pointing from the light. The right-hand tendril, whilst 
converging, zigzagged much more than the one figured. Both 
tracings showed that the apheliotropic movement was a modi- 
fied form of circumnutation. 

Cyclamen Persicum. — Whilst this plant is in flower the peduncles 
stand upright, but their uppermost part is hooked so that the 
flower itself hangs downwards. As soon as the pods begin to 
swell, the peduncles increase much in length and slowly curve 
downwards, but the short, upper, hooked part straightens itself. 
Ultimately the pods reach the ground, and if this is covered 
with moss or dead leaves, they bury themselves. We have often 
seen saucer-like depressions formed by the pods in damp sand 
or sawdust ; and one pod ( • 3 of inch in diameter) buried itself 
in sawdust for three-quarters of its length. f We shall have 
occasion hereafter to consider the object gained by this burying 
process. The peduncles can change the direction of their cur- 
vature, for if a pot, with plants having their peduncles already 
bowed downwards, be placed horizontally, they slowly bend 
at right angles to their former direction towards the centre of 
the earth. We therefore at first attributed the movement to 
geotropism ; but a pot which had lain horizontally with the pods 



* • The Movements and Habits tanic Garden,' Canto., iii. p. 126), 

of Climbing Plants,' 1875, p. 97. the pods forcibly penetrate the 

t The peduncles of several earth. See also Grenier and 

other species of Cyclamen twist Godron, * Fl ;>re de France,' torn ii 

themselves into a spire, and ao p. 459. 
uurding to Erasmus Darwin C Bo- 



134 



MODIFIED CIKCUMNTJTATIOK 



Chap. VIII. 



all painting to the ground, was reversed, being still kept hori- 
zontal, so that the pods now pointed directly upwards ; it was 
then placed in a dark cupboard, but the pods still pointed up- 
wards after four days and nights. The pot, in the same position, 
was next brought back into the light, and after two days there 
was some bending downwards of the peduncles, and on the fourth 
day two of them pointed to the centre of the earth, as did the 
others after an additional day or two. Another plant, in a pot 
which had always stood upright, was left in the dark cupboard 
for six days ; it bore 3 peduncles, and only one became within this 

Fig. 179. 




Cyclamen Persicum: downward apheliotropic movement of a flower-peduncle, 
greatly magnified (about 47 times ?), traced on a horizontal glass from 
1 p.m. Feb. 18th to 8 A.M. 21st. 



time at all bowed downwards, and that doubtfully. The weight, 
therefore, of the pods is not the cause of the bending down. 
This pot was then brought back into the light, and after three 
days the peduncles were considerably bowed downwards. We 
are thus led to infer that the downward curvature is due to 
apheliotropism ; though more trials ought to have been made. 

In order to observe the nature of this movement, a peduncle 
bearing a large pod which had reached and rested on the 
ground, was lifted a little up and secured to a stick. A filament 
was fixed across the pod with a mark beneath, and its move* 



CiiAr. VIII. i.PHELIOTBOPIStf. 435 

ment, greatly magnified, was traced on a horizontal glass during 
67 h. The plant was illuminated during the day from above. A 
copy of the tracing is given on p. 434 (Fig. 179) ; and there can 
be no doubt that the descending movement is one of modified 
circumn«utation, but on an extremely small scale. The observa- 
tion was repeated on another pod, which had partially buried 
itself in sawdust, and which was lifted up a quarter of an inch 
above the surface; it described three very small circles in 24 h. 
Considering the great length and thinness of the peduncles 
and the lightness of the pods, we may conclude that they 
would not be able to excavate saucer-like depressions in sand 
or sawdust, or bury themselves in moss, &c, unless they were 
aided by their continued rocking or circumnutating move- 
ment. 

Relation hetiveen Gir cum nutation and Heliotrojnsm. — 
Any one who will look at the foregoing diagrams, 
showing the movements of the stems of various plants 
towards a lateral and more or less dimmed light, will 
be forced to admit that ordinary circumnutation and 
heliotropism graduate into one another. When a 
plant is exposed to a dim lateral light and continues 
during the whole day bending towards it, receding 
late in the evening, the movement unquestionably is 
one of heliotropism. Now, in the case of Tropaeolura 
(Fig. 175) the stem or epicotyl obviously chcumnu- 
tated during the whole day, and yet it continued at 
the same time to move heliotropically ; this latter 
movement being effected by the apex of each succes- 
sive elongated figure or ellipse standing nearer to 
the light than the previous one. In the case of 
Cassia (Fig. 177) the comparison of the movement ot 
the hypocotyl, when exposed to a dim lateral light and 
to darkness, is very instructive ; as is that between 
the ordinary circumnutating movement of a seedling 
Brassica (Figs. 172, 173), or that of Phalaris (Figs. 
49,~174), and their heliotropic movement towards a 
window protected by blinds. In both these cases 



±3(3 RELATION BETWEEN Chap. VIII 

and in many others, it was interesting to notice Low 
gradually the stems began to circumnutate as the 
light waned in the evening. We have therefore many 
kinds of gradations from a movement towards the light, 
which must be considered as one of circumnutation 
very slightly modified and still consisting of ellipses 
or circles, — though a movement more or less strongly 
zigzag, with loops or ellipses occasionally formed, — to 
a nearly straight, or even quite straight, heliotropic 
course. 

A plant, when exposed to a lateral light, though 
this may be bright, commonly moves at first in a 
zigzag line, or even directly from the light ; and 
this no doubt is due to its circumnutating at the 
time in a direction either opposite to the source of 
the light, or more or less transversely to it. As soon, 
however, as the direction of the circumnutating move- 
ment nearly coincides with that of the entering light, 
the plant bends in a straight course towards the light, 
if this is bright. The course appears to be rendered 
more and more rapid and rectilinear, in accordance with 
the degree of brightness of the light — firstly, by the 
longer axes of the elliptical figures, which the plant 
continues to describe as long as the light remains very 
dim, being directed more or less accurately towards 
its source, and by each successive ellipse being de- 
scribed nearer to the light. Secondly, if the light 
is only somewhat dimmed, by the acceleration and 
increase of the movement towards it, and by the 
retardation or arrestment of that from the light, some 
lateral movement being still retained, for the light 
will interfere less with a movement at right angles 
to its direction, than with one in its own direction.* 



" In bis paper, ' Ut her oitlio- tleile' (' Avbeiten des Bot. Inst 
trope und plagiotrope Pfianzen- in Wurzburg,' Band ii. Heft ii 



Chap. VIII. CIRCUMNUTATION AND HELIOS ROPISM. 437 

The result is that the course is rendered more or less 
zigzag aud unequal in rate. Lastly, when the light 
is very bright all lateral movement is lost ; and the 
whole energy of the plant is expended in renderiug 
the circumnutating movement rectilinear and rapid in 
oue direction alone, namely, towards the light. 

The common view seems to be that heliotropism is 
a quite distinct kind of movement from circumnuta- 
tion ; and it may be urged that in the foregoing 
diagrams we see heliotropism merely combined with, 
or superimposed on, circumnutation. But if so, it must 
be assumed that a bright lateral light completely 
stops circumnutation, for a plant thus exposed moves 
in a straight line towards it, without describing any 
ellipses or circles. If the light be somewhat obscured, 
though amply sufficient to cause the plant to bend 
towards it, we have more or less plain evidence of still- 
continued circumnutation. It must further be assumed 
that it is only a lateral light which has this extraor- 
dinary power of stopping circumnutation, for we know 
that the several plants above experimented on, and 
all the others which were observed by us whilst grow- 
ing, continue to circumnutate, however bright the light 
may be, if it comes from above. Nor should it be 
forgotten that in the life of each plant, circumnuta- 
tion precedes heliotropism, for hypocotyls, epicotyls, 
and petioles circumnutate before they have broken 
through the ground and have ever felt the influence of 
light. 

We are therefore fully justified, as it seems to us, in 
believing that whenever light enters laterally, it is the 



1879), Sachs has discussed the the organs of plants stand with 

manner in which geotropism and respect to the direction of the 

heliotropism are affected by dif- incident force, 
ferences in the angles at which 



£38 MODIFIED CIECUMNUTATION. Chap. VIIL 

movement of circumnutation which gives rise to, or is 
converted into, heliotropism and apheliotropism. On 
this view we need not assume against all analogy that 
a lateral light entirely stops circumnntation ; it merely 
excites the plant to modify its movement for a time 
in a beneficial manner. The existence of every pos- 
sible gradation, between a straight course towards a 
lateral light and a conrse consisting of a series of loops 
or ellipses, becomes perfectly intelligible. Finally, 
the conversion of circnmnntation into heliotropism or 
apheliotropism, is closely analogous to what takes place 
with sleeping plants, which during the daytime de- 
scribe one or more ellipses, often moving in zigzag lines 
and making little loops ; for when they begin in the 
evening to go to sleep, they likewise expend all their 
energy in rendering their course rectilinear and rapid. 
In the case of sleep-movements, the exciting or regu- 
lating cause is a difference in the intensity of the 
light, coming from above, at different periods of the 
twenty-four hours ; whilst with heliotropic and aphe- 
liotropic movements, it is a difference in the intensity 
of the light on the two sides of the plant. 

Tramvcrsal-lieliotropismus {of Frank *) or Diahelio- 
tropism. — The cause of leaves placing themselves 
more or less transversely to the light, with their 
upper surfaces directed towards it, has been of late 
the subject of much controversy. We do not here 
refer to the object of the movement, which no doubt 
is that their upper surfaces may be fully illuminated, 
but the means by which this position is gained. 
Hardly a better or more simple instance can be given 



* 'Die natiirlioliK Wugercchte Frnge liber Transversal Geo-und 

ltichtung von Pnanzenth< ilen,' Heliotropismus," ' Bot. Zeitung, 

18 '0 See also suue interesting 1873, p. 17 et seq. 
aiticles by the same author, " Zux 



Chap. VIII. DIAHELIOTROPISM. 439 

of diaheliotropism than that offered by many seed- 
lings, the cotyledons of which are extended hori- 
zontally. When they first burst from their seed-coats 
they are in contact and. stand, in various positions, 
often vertically upwards ; they soon diverge, and this 
is effected by epinasty, which, as we have seen, is a 
modified form of circumnutation. After they have 
diverged to their full extent, they retain nearly the 
same position, though brightly illuminated all day 
long from above, with their lower surfaces close to the 
ground and thus much shaded. There is therefore a 
great contrast in the degree of illumination of their 
upper and lower surfaces, and if they were heliotropic 
they would bend quickly upwards. It must not, how- 
ever, be supposed that such cotyledons are immovably 
fixed in a horizontal position. When seedlings are 
exposed before a window, their hypocotyls, which are 
highly heliotropic, bend quickly towards it, and the 
upper surfaces of their cotyledons still remain ex- 
posed at right angles to the light ; but if the hypo- 
cotyl is secured so that it cannot bend, the cotyledons 
themselves change their position. If the two are 
placed in the line of the entering light, the one 
furthest from it rises up and that nearest to it often 
sinks down ; if placed transversely to the light, they 
twist a little laterally ; so that in every case they 
endeavour to place their upper surfaces at right angles 
to the light. So it notoriously is with the leaves on 
plants nailed against a wall, or grown in front of a 
window. A moderate amount of light suffices to in- 
duce such movements ; all that is necessary is that the 
light should steadily strike the plants in an oblique 
direction. With respect to the above twisting move- 
ment of cotyledons, Frank has given many and much 
more striking instances in the case of the leaves on 

29 



440 MODIFIED CIKCUMNUTATION. Chap. Vm 

branches which had been fastened in various positions 
or turned upside down. 

In our observations on the cotyledons of seedling 
plants, we often felt surprise at their persistent hori- 
zontal position during the day, and were convinced 
before we had read Frank's essay, that some special 
explanation was necessary. De Vries has shown* 
that the more or less horizontal position of leaves is 
in most cases influenced by epinasty, by their own 
weight, and by apogeotropism. A young cotyledon 
or leaf after bursting free is brought down into its 
proper position, as already remarked, by epinasty, 
which, according to De Vries, long continues to act 
on the midribs and petioles. Weight can hardly be 
influential in the case of cotyledons, except in a few 
cases presently to be mentioned, but must be so with 
large and thick leaves. With respect to apogeotropism, 
De Vries maintains that it generally comes into play, 
and of this fact we shall presently advance some 
indirect evidence. But over these and other constant 
forces we believe that there is in many cases, but we 
do not say in all, a preponderant tendency in leaves 
and cotyledons to place themselves more or less trans- 
versely with respect to the light. 

In the cases above alluded to of seedlings exposed 
to a lateral light with their hypocotyls secured, it is 
impossible that epinasty, weight and apogeotropism, 
either in opposition or combined, can be the cause of 
the rising of one cotyledon, and of the sinking of the 
other, since the forces in question act equally on both : 
and since epinasty, weight and apogeotropism all act 
in a vertical plane, they cannot cause the twisting of 
the petioles, which occurs in seedlings under the 



* 'Arbeikm des Bot. Institute in Wiirz'mrg,' Heft. ii. 1872, pp. 
223-277. 



Chap. VIII. DIAHELIOTROPISM. 441 

above conditions of illumination. All thest movements 
evidently depend in some manner on the obliquity of 
the light, but cannot be called heliotropic, as this 
implies bending towards the light ; whereas the coty- 
ledon nearest to the light bends in an opposed direc- 
tion or downwards, and both place themselves as nearly 
as possible at right angles to the light. The move- 
ment, therefore, deserves a distinct name. As coty- 
ledons and leaves are continually oscillating up and 
down, and yet retain all day long their proper position 
with their upper surfaces directed transversely to the 
light, and if displaced reassume this position, dia- 
heliotropism must be considered as a modified form of 
circumnutation. This was often evident when the 
movements of cotyledons standing in front of a window 
were traced. We see something analogous in the case 
of sleeping leaves or cotyledons, which after oscillating 
up and down during the whole day, rise into a vertical 
position late in the evening, and on the following 
morning sink clown again into their horizontal or dia- 
heliotropic position, in direct opposition to heliotro- 
pism. This return into their diurnal position, which 
often requires an angular movement of 90°, is analo- 
gous to the movement of leaves on displaced branches, 
which recover their former positions. It deserves 
notice that any force such as apogeotropism, will act 
with different degrees of power* in the different posi- 
tions of those leaves or cotyledons which oscillate 
largely up and down during the day ; and yet they 
recover their horizontal or diaheliotropic position. 

We may therefore conclude that diaheliotropic 
movements cannot be fully explained by the direct 
action of light, gravitation, weight, &c, any more 



* See former note, in reference to Sachs' remarks on this sr bject. 



±42 MODIFIED CIKCUMNUTATION. Ch*i\ VIII 

than can the nyctitropic movements of cotyledons 
and leaves. In the latter case they place themselves 
so that their upper surfaces may radiate at night 
as little as possible into open space, with the upper 
surfaces of the opposite leaflets often in contact. These 
movements, which are sometimes extremely complex, 
are regulated, though not directly caused, by the alter- 
nations of light and darkness. In the case of diahelio- 
tropism, cotyledons and leaves place themselves so 
that their upper surfaces may be exposed to the light, 
and this movement is regulated, though not directly 
caused, by the direction whence the light proceeds. In 
both cases the movement consists of circumnutation 
modified by innate or constitutional causes, in the 
same manner as with climbing plants, the circumnu- 
tation of which is increased in amplitude and rendered 
more circular, or again with very young cotyledons 
and leaves which are thus brought down into a hori- 
zontal position by epinasty. 

We have hitherto referred only to those leaves and 
cotyledons which occupy a permanently horizontal 
position ; but many stand more or less obliquely, and 
some few upright. The cause of these differences of 
position is not known ; but in accordance with Wiesner's 
views, hereafter to be given, it is probable that some 
leaves and cotyledons would suffer, if they were fully 
illuminated by standing at right angles to the light. 

We have seen in the second and fourth chapters 
that those cotyledons and leaves which do not alter 
their positions at night sufficiently to be said to sleep, 
commonly rise a little in the evening and fall again 
on the next morning, so that they stand during the 
night at a rather higher inclination than during the 
middle of the day. It is incredible that a rising 
movement of 2° or 3°, or even of 10° or 20°, can be oi 






Chap. VLLI. DIAHELIOTKOPISM. 4'1'J 

any service to the plant, so as to have been specially 
acquired. It must be the result of some periodical 
change in the conditions to which they are subjected, 
and there can hardly be a doubt that this is the daily 
alternations of light and darkness. De Vries states in 
the paper before referred to, that most petioles and 
midribs are apogeotropic ;* and apogeotropism would 
account for the above rising movement, which is com- 
mon to so many widely distinct species, if we suppose it 
to be conquered by diaheliotropism during the middle 
of the day, as long as it is of importance to the plant 
that its cotyledons and leaves should be fully exposed 
to the light. The exact hour in the afternoon at which 
they begin to bend slightly upwards, and the extent of 
the movement, will depend on their degree of sen- 
sitiveness to gravitation and on their power of resist- 
ing its action during the middle of the day, as well as 
on the amplitude of their ordinary circumnutating 
movements ; and as these qualities differ much in dif- 
ferent species, we might expect that the hour in the 
afternoon at which they begin to rise would differ 
much in different species, as is the case. Some other 
agency, however, besides apogeotropism, must come 
into play, either directly or indirectly, in this upward 
movement. Thus a young bean (Vicia faba), growing 
in a small pot, was placed in front of a window in a 
klinostat ; and at night the leaves rose a little, although 



* According to Frank ('Die organs have been long kept in the 
nat. Wagerechte Eichtung von da-k, the amount of water and of 
Prtanzentheilen.' 1870, p. 46) the mineral matter which they con- 
root-leaves of many pLants, kept tain is so much altered, and their 
in darkness, rise up and even be- regular growth is so much dis- 
come vertical ; and so it is in some turbed, that it is perhaps rash to 
cases with shoots. (See Eauwen- infer from their movements what 
hoff, 'Archives Neerlandaises,' would occur under normal con- 
tom. xii. p. 32.) These movements ditions. (See Godlewski, 'Bot 
indicate apogeotiopism ; but when Zeitung,' Feb. 14th, 1879.) 



144 MODIFIED CIRCUMNUTATION. Chap. VIII 

the action of apogeotropism was quite eliminated. 
Nevertheless, they did not rise nearly so much at 
night, as when subjected to apogeotropism. Is it 
not possible, or even probable, that leaves and coty- 
ledons, which have moved upwards in the evening 
through the action of apogeotropism during countless 
generations, may inherit a tendency to this movement ? 
We have seen that the hypocotyls of several Legu- 
minous plants have from a remote period inherited a 
tendency to arch themselves ; and we know that the 
sleep-movements of leaves are to a certain extent 
inherited, independently of the alternations of light 
and darkness. 

In our observations on the circumnutation of those 
cotyledons and leaves which do not sleep at night, we 
met with hardly any distinct cases of their sinking 
a little in the evening, and rising again in the morn- 
ing, — that is, of movements the reverse of those just 
discussed. We have no doubt that such cases occur, 
inasmuch as the leaves of many plants sleep by 
sinking vertically downwards. How to account for the 
few cases which were observed must be left doubtful. 
The young leaves of Cannabis sativa sink at night 
between 30° and 40° beneath the horizon ; and Kraus 
attributes this to epinasty in conjunction with the 
absorption of water. Whenever epinastic growth is 
vigorous, it might conquer diaheliotropism in the 
evening, at which time it would be of no import- 
ance to the plant to keep its leaves horizontal. 
The cotyledons of Anoda Wrightii, of one variety of 
Grossypium, and of several species of Ipomoea, remain 
horizontal in the evening whilst they are very young ; 
as they grow a little older they curve a little down- 
wards, and when large and heavy sink so much that 
they come under our definition of sleep. In the case of 



Chap. VIII. PARAHELIOTROPISM. 445 

the Anoda and of some species of Ipomoea, it was proved 
that the downward movement did not depend on the 
weight of the cotyledons ; but from the fact of the move- 
ment being so much more strongly pronounced after 
the cotyledons have grown large and heavy, we may 
suspect that their weight aboriginally played some part 
in determining that the modification of the circum- 
nutating movement should be in a downward direction. 
The so-called Diurnal Sleep of Leaves, or Parahelio- 
tropism. — This is another class of movements, dependent 
on the action of light, which supports to some extent 
the belief that the movements above described are 
only indirectly due to its action. We refer to the 
movements of leaves and cotyledons which when 
moderately illuminated are diahelio tropic ; but which 
change their positions and present their edges to the 
light, when the sun shines brightly on them. These 
movements have sometimes been called diurnal sleep, 
but they differ wholly with respect to the object 
gained from those properly called nyctitropic ; and in 
some cases the position occupied during the day is the 
reverse of that during the night. 

It has long been known* that when the sun shines brightly 
on the leaflets of Eobinia, they rise up and present their edges 
to the light ; whilst their position at night is vertically down- 
wards. We have observed the same movement, when the 
sun shone brightly on the leaflets of an Australian Acacia. 
Those of Am/phicarpma monoica turned their edges to the sun ; 
and an analogous movement of the little almost rudimentary 
basal leaflets of Mimosa albida was on one occasion so rapid that 
it could be distinctly seen through a lens. The elongated, uni- 
foliate, first leaves of Phaseolus Roxburghii stood at 7 a.m. at 20° 
above the horizon, and no doubt they afterwards sank a little 
lower. At noon, after having been exposed for about 2 h. to 



* Pfeffer gives the names and dates of sevoral ancient writers in his 
Die Periodischen Bewegungen,' 1875, p. 62. 



446 MODIFIED CIRCUMNUTATION. Chap. VHT. 

a bright sun, they stood at 56° above the horizon ; they were 
then protected from the rays of the sun, but were left well 
illuminated from above, and after 30 m. they had fallen 40°, for 
they now stood at only 16° above the horizon. Some young 
plants of Fhaseolus Hernandesii had been exposed to the same 
bright sunlight, and their broad, unifoliate, first leaves now 
stood up almost or quite vertically, as did many of the leaflets 
on the trifoliate secondary leaves ; but some of the leaflets had 
twisted round on their own axes by as much as 90° without 
rising, so as to present their edges to the sun. The leaflets on 
the same leaf sometimes behaved in these two different manners, 
but always with the result of being less intensely illuminated. 
These plants were then protected from the sun, and were looked 
at after lh h. ; and now all the leaves and leaflets had re- 
assumed their ordinary sub-horizontal positions. The copper- 
coloured cotyledons of some seedlings of Cassia mimosoides were 
horizontal in the morning, but after the sun had shone on 
them, each had risen 45^° above the horizon. The movement 
in these several cases must not be confounded with the sudden 
closing of the leaflets of Mimosa pudica, which may sometimes 
be noticed when a plant which has been kept in an obscure 
place is suddenly exposed to the sun ; for in this case the light 
seems to act, as if it were a touch. 

From Prof. Wiesner's interesting observations, it is probable 
that the above movements have been acquired for a special 
purpose. The chlorophyll in leaves is often injured by too 
intense a light, and Prof. Wiesner * believes that it is protected 
by the most diversified means, such as the presence of hairs, 
colouring matter, &c, and amongst other means by the leaves 
presenting their edges to the sun, so that the blades then 
receive much less light. He experimented on the young leaflets 
of Eobinia, by fixing them in such a position that they could 
not escape being intensely illuminated, whilst others were 
allowed to place themselves obliquely ; and the former began to 
suffer from the light in the course of two days. 

In the cases above given, the leaflets move either upwards 



* 'Die N'aturlichen Einrich- the action of concentrated light 

tungen zum Schutze des Chloro- from the sun, in the presence of 

phylls,' &c, 1876. Pringsheim oxygen. See, also, Stahl on th6 

lias recently observed under the protection of chlorophyll from 

microscope the destruction of intense light, in 'Bot. Zeitung, 

chlorophyll in a few minutes by 1880. 



Chap. VIII. PARAHELIOTROriSM. 447 

or twist laterally, so as to place their edges in the direction of the 
sun's light; but Cohn long ago observed that the leaflets of 
Oxalis bend downwards when fully exposed to the sun. We 
witnessed a striking instance of this movement in the very 
large leaflets of 0. Urteyesii. A similar movement may fre- 
quently be observed with the leaflets of Averrhoa bilimbi (a 
member of the Oxalidse) ; and a leaf is here represented (Fig. 
180) on which the sun had shone. A diagram (Fig. 134) was 
given in the last chapter, representing the oscillations by which 
a leaflet rapidly descended under these circumstances ; and the 
movement may be teen closely to resemble that (Fig. 133) bv 

Fig-. 180. 




Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine* 
but the leaflets are sometimes more depressed than is here shown. 
Figure much reduced. 

which it assumed its nocturnal position. It is an interesting 
fact in relation to our present subject that, as Prof. Batalin 
informs us in a letter, dated February, 1879, the leaflets of 
Oxalis acetosella may be daily exposed to the sun during many 
weeks, and they do not suffer if they are allowed to depress 
themselves ; but if this be prevented, they lose their colour and 
wither in two or three days. Yet the duration of a leaf is about 
two months, when subjected only to diffused light; and in this 
case the leaflets never sink downwards during the day 

As the upward movements of the leaflets of Eobinia, 
and the downward movements of those of Oxalis, have 
been proved to be highly beneficial to these plants 
when subjected to bright sunshine, it seems probable 
that they have been acquired for the special purpose 
of avoiding too intense an illumination. As it would 
have been very troublesome in all the above cases to 



148 MODIFIED CIRCUMNUTATIOK Chap. Via 

have watched for a fitting opportunity and to have 
traced the movement of the leaves whilst they were 
fully exposed to the sunshine, we did not ascertain 
whether paraheliotropism always consisted of modi- 
tied circumnutation ; but this certainly was the case 
with the Averrhoa, and probably with the other species, 
©3 their leaves were continually circumnutatin^. 



Chat. IX. SENSITIVENESS TO LIGHT. 440 



CHAPTEK IX. 

Sensitiveness of Plants to Light: its transmitted effects. 

Uses of heliotropism — Insectivorous and climbing plants not heliotropic 
— Same orjjan helii ■tropic at one age and not at another — Extra- 
ordinary sensitiveness of some plants to light — The effects of light dc 
not correspond with its intensity— Effects of previous illumination 
— Time required for the action of light— After-effects of light — 
ApogeotropLm acts as soon as light fails — Accuracy with which 
plants bend to the light — This dependent on the illumination of 
one whole side of the part — Localised sensitiveness to light and its 
transmitted effects — Cotyledons of 1 halaris, manner of bending — 
Results of the exclusion of light from their tips — Effects trans- 
mitted beneath the surface of the ground — Lateral illumination of 
the tip determines the direction of the curvature of the base— Coty- 
ledons of A vena, curvature of basal part due to the illumination of 
upper part — Similar results with the hypocotyls of Brassica and 
Beta — Radicles of Sinapis apheliotropic, due to the sensitiveness of 
their tips — Concluding remarks and summary of chapter — Means 
by which circumnutation has been converted into heliotropism or 
apheliotropibrn. 

No one can look at the plants growing on a bank or 
on the borders of a thick wood, and doubt that the 
young stems and leaves place themselves so that the 
leaves may be well illuminated. They are thus enabled 
to decompose carbonic acid. But the sheath-like coty- 
ledons of some Graminese, for instance, those of Pha- 
laris, are not green and contain very little starch ; 
from which fact we may infer that they decompose 
little or no carbonic acid. Nevertheless, they are ex- 
tremely heliotropic ; and this probably serves them in 
another way, namely, as a guide from the buried seeds 
through fissures in the ground or through overlying 
masses of vegetation, iuto the light and air. This view 



£50 SENSITIVENESS TO LIGHT. Chap. IX. 

is strengthened by the fact that with Phalaris and 
Avena the first trne leaf, which is bright green and nc 
doubt decomposes carbonic acid, exhibits hardly a 
trace of heliotropism. The heliotropic movements of 
many other seedlings probably aid them in like 
manner in emerging from the ground ; for apogeo- 
tropism by itself would blindly guide them v.pwards, 
against any overlying obstacle. 

Heliotropism prevails so extensively among the 
higher plants, that there are extremely few, of which 
some part, either the stem, flow T er-peduncle, petiole, 
or leaf, does not bend towards a lateral light. 
Drosera rotundifolia is one of the few T plants the 
leaves of which exhibit no trace of heliotropism. Nor 
could we see any in Dionsea, though the plants were 
not so carefully observed. Sir J. Hooker exposed the 
pitchers of Sarracenia for some time to a lateral light, 
but they did not bend towards it.* We can understand 
the reason why these insectivorous plants should not 
be heliotropic, as they do not live chiefly by decom- 
posing carbonic acid ; and it is much more important 
to them that their leaves should occupy the best 
position for capturing insects, than that they should 
be fully exposed to the light. 

Tendrils, which consist of leaves or of other organs 
modified, and the stems of twining plants, are, as 
Mohl long ago remarked, rarely heliotropic ; and here 
again we can see the reason why, for if they had 
moved towards a lateral light they would have been 
drawn away from their supports. But some tendrils are 
apheliotropic, for instance those of Bignonia cajpreolata 



* According to F. Kurtz (' Ver- tonia Calif omica are strongly 

handl. di s Bot. Vereins der Pro- apheliotropic. We failed to detect 

vinz Brandenburg,' Bd. xx. 1JS78) this movement in a plant which 

the leaves or pitchers of Darling- we possessed for a short time. 






Chap, IX. SENSITIVENESS TO LIGHT. '151 

and of Smilax aspera ; and the stems of some plants 
which climb by rootlets, as those of the Ivy and Tecoma 
radicans, are likewise apheliotropic, and they thus find 
a support. The leaves, on the other hand, of most 
climbing plants are heliotropic ; but we could detect 
no signs of any such movement in those of Mutiaia 
clematis. 

As heliotropism is so widely prevalent, and as 
twining plants are distributed throughout the whole 
vascular series, the apparent absence of any tendency 
in their stems to bend towards the light, seemed to 
us so remarkable a fact as to deserve further in- 
vestigation, for it implies that heliotropism can be 
readily eliminated. When twining plants are exposed 
to a lateral light, their stems go on revolving or cir- 
cumnutating about the same spot, without any evident 
deflection towards the light ; but we thought that 
we might detect some trace of heliotropism by com- 
paring the average rate at which the stems moved to 
and from the light during their successive revolutions.* 
Three young plants (about a foot in height) of Ipomoea 
cmrulea and four of I. purpurea, growing in separate 
pots, were placed on a bright day before a north-east 
window in a room otherwise darkened, with the tips 
of their revolving stems fronting the window. When 
the tip of each plant pointed directly from the window, 
and when again towards it, the times w T ere recorded. 
This was continued from 6.45 a.m. till a little after 
2 p.m. on June 17th. After a few observations we 
concluded that we could safely estimate the time 



* Some erroneous statements number of observations, for we did 
are unfortunately given on this not then know at how unequal 
subject, in 'The Movements and a rate the stems and tetidrils of 
Habits of Climb in % PJants,' 1875, climbing plants rometimes travel 
pp. 28, 32, 40, and 53. Conclusions in different parts of the same re- 
were drawn from an insufficient volution. 



£52 SENSITIVENESS TO LIGHT. Chap. IX. 

taken by each semicircle, within a limit of error of at 
most 5 minutes. Although the rate of movement in 
different parts of the same revolution varied greatly, 
yet 22 semicircles to the light were completed, each 
on an average in 73*95 minutes; and 22 semicircles 
from the light each in 73'5 minutes. It may, there- 
fore, be said that they travelled to and from the light 
at exactly the same average rate ; though probably 
the accuracy of the result was in part accidental. In 
the evening the stems were not in the least deflected 
towards the window. Nevertheless, there appears to 
exist a vestige of heliotropism, for with 6 out of the 
7 plants, the first semicircle from the light, described 
in the early morning after they had been subjected to 
darkness during the night and thus probably rendered 
more sensitive, required rather more time, and the first 
semicircle to the light considerably less time, than the 
average. Thus with all 7 plants, taken together, the 
mean time of the first semicircle in the morning from 
the light, was 76*8 minutes, instead of 73*5 minutes, 
which is the mean of all the semicircles during the 
day from the light ; and the mean of the first semi- 
circle to the light was only 63*1, instead of 73*95 
minutes, which was the mean of all the semicircles 
during the day to the light. 

Similar observations were made on Wistaria Sinensis, 
and the mean of 9 semicircles from the light was 
117 minutes, and of 7 semicircles to the light 122 
minutes, and this difference does not exceed the pro- 
bable limit of error. During the three days of expo- 
sure, the shoot did not become at all bent towards the 
window before which it stood. In this case the first 
semicircle from the light in the early morning of each 
day, required rather less time for its performance thar 
did the first semicircle to the light; and this result, 






Ciikv. IX. SENSITIVENESS 'JO LIGHT. 453 

if not accidental, appears to indicate that the shoota 
retain a trace of an original apheliotropic tendency. 
With Lonicera brachijpoda the semicircles from and to 
the light differed considerably in time ; for 5 semi- 
circles from the light required on a mean 202*4 
minutes, and 4 to the light, 229*5 minutes ; but the 
shoot moved very irregularly, and under these circum- 
stances the observations were much too few. 

It is remarkable that the same part on the same 
plant may be affected by light in a widely different 
manner at different ages, and as it appears at different 
seasons. The hypocotyledonous steins of Ipomoea 
cserulea and purpurea are extremely heliotropic, whilst 
the stems of older plants, only about a foot in height, 
are, as we have just seen, almost wholly insensible to 
light. Sachs states (and we have observed the same 
fact) that the hypocotyls of the Ivy (Hedera helix) are 
slightly heliotropic ; whereas the stems of plants grown 
to a few inches in height become so strongly aphelio- 
tropic, that they bend at right angles away from the 
light. Nevertheless, some young plants which had 
behaved in this manner early in the summer again 
became distinctly heliotropic in the beginning of 
September ; and the zigzag courses of their stems, as 
they slowly curved towards a north-east window, were 
traced during 10 days. The steins of very young 
plants of Tropseolum majus are highly heliotropic, whilst 
those of older plants, according to Sachs, are slightly 
apheliotropic. In all these cases the heliotropism of 
the very young steins serves to expose the cotyledons, 
or when the cotyledons are hypogean the first true 
leaves, fully to the light ; and the loss of this power 
by the older stems, or their becoming apheliotropic, 
is connected with their habit of climbing. 

Most seedling plants are strongly heliotropic, and 



154 SENSITIVENESS TO LIGHT. Chap. IX 

it is no doubt a great advantage to them in their 
struggle for life to expose their cotyledons to the 
light as quickly and as fully as possible, for the sake 
of obtaining carbon. It has been shown in the first 
chapter that the greater number of seedlings circum- 
nutate largely and rapidly ; and as heliotropism con- 
sists of modified circumnutation, we are tempted to 
look at the high development of these two powers in 
seedlings as intimately connected. Whether there are 
any plants which circumnutate slowly and to a small 
extent, and yet are highly heliotropic, we do not 
know ; but there are several, and there is nothing 
surprising in this fact, which circumnutate largely and 
are not at all, or only slightly, heliotropic. Of such 
cases Brosera rotundifolia offers an excellent instance, 
The stolons of the strawberry circumnutate almost 
like the stems of climbing plants, and they are not at 
all affected by a moderate light ; but when exposed 
late in the summer to a somewhat brighter light they 
were slightly heliotropic ; in sunlight, according to 
De Vries, they are apheliotropic. Climbing plants 
circumnutate much more widely than any other plants, 
yet they are not at all heliotropic. 

Although the stems of most seedling plants are 
strongly heliotropic, some few are but slightly helio- 
tropic, without our being able to assign any reason. 
This is the case with the hypocotyl of Cassia tora, and 
we were struck with the same fact with some other 
seedlings, for instance, those of Reseda odor at a. With 
respect to the degree of sensitiveness of the more 
sensitive kinds, it was shown in the last chapter that 
seedlings of several species, placed before a north-east 
window protected by several blinds, and exposed in 
the rear to the diffused light of the room, moved 
with unerring certainty towards the window, although 






Chai\ IX. SENSITIVENESS TO LIGHT. 455 

it was impossible to judge, excepting by the shadow 
cast by an upright pencil on a white card, on which 
side most light entered, so that the excess on one side 
must have been extremely small. 

A pot with seedlings of Phalaris Ganariensis, which 
had been raised in darkness, was placed in a com- 
pletely darkened room, at 12 feet from a very small 
lamp. After 3 h. the cotyledons were doubtfully 
curved towards the light, and after 7 h. 40 m. from 
the first exposure, they were all plainly, though 
slightly, curved towards the lamp. Now, at this dis- 
tance of 12 feet, the light was so obscure that we could 
not see the seedlings themselves, nor read the large 
Roman figures on the white face of a watch, nor see a 
pencil line on paper, but could just distinguish a line 
made with Indian ink. It is a more surprising fact 
that no visible shadow was cast by a pencil held 
upright on a white card; the seedlings, therefore, 
were acted on by a difference in the illumination of 
their two sides, which the human eye could not dis- 
tinguish. On another occasion even a less degree of 
light acted, for some cotyledons of Phalaris became 
slightly curved towards the same lamp at a distance 
of 20 feet; at this distance we could not see a cir- 
cular dot 2*29 mm. (-09 inch) in diameter made with 
Indian ink on white paper, though we could just see a 
dot 3*56 mm. (*14 inch) in diameter; yet a dot of 
the former size appears large when seen in the light.* 

We next tried how small a beam of light would act ; 
as this bears on light serving as a guide to seedlings 
whilst they emerge through fissured or encumbered 
ground. A pot with seedlings of Phalaris was cove-ed 



* Strasburger says (' Wirkung Haematococcus moved to a light 

des Lichtes auf Sciiwarrasporen,' which only just sufficed to alio* 

1878, p, 52), that the spores of middle-sized type to be read. 

30 



±56 SENSITIVENESS TO LIGHT. Ctiap. IS. 

by a tin-vessel, having on one side a circular hole 
123 mm. in diameter (i.e. a little less than the -n-^-tk oi 
an inch) ; and the box was placed in front of a paraffin 
lamp and on another occasion in front of a window ; 
and both times the seedlings were manifestly bent 
after a few hours towards the little hole. 

A more severe trial was now made ; little tubes of 
very thin glass, closed at their upper ends and coated 
with black varnish, were slipped over the cotyledons 
of Phalaris (which had germinated in darkness) and 
just fitted them. Narrow stripes of the varnish had 
been previously scraped off one side, through which 
alone light could enter ; and their dimensions were 
afterwards measured under the microscope. As a 
control experiment, similar unvarnished and trans- 
parent tubes were tried, and they did not prevent the 
cotyledons bending towards the light. Two cotyledons 
were placed before a south-west window, one of which 
was illuminated by a stripe in the varnish, only *004 
inch (0*1 mm.) in breadth and *016 inch (0*4 mm.) in 
length ; and the other by a stripe '008 inch in breadth 
and '06 inch in length. The seedlings were examined 
after an exposure of 7 h. 40 m., and were found to be 
manifestly bowed towards the light. Some other coty- 
ledons were at the same time treated similarly, ex- 
cepting that the little stripes were directed not to the 
sky, but in such a manner that they received only the 
diffused light from the room ; and these cotyledons did 
not become at all bowed. Seven other cotyledons were 
illuminated through narrow, but comparatively long, 
cleared stripes in the varnish — namely, in breadth 
between "01 and "026 inch, and in length between "15 
and '3 inch ; and these all became bowed to the side, 
by which light entered through the stripes, whether 
these were directed towards the sky or to one side oi 



Chap. IX. SENSITIVENESS TO LIGHT. 457 

the room. That light passing through a hole only 
004 inch in breadth by *016 in length, should induce 
curvature, seems to us a surprising fact. 

Before we knew how extremely sensitive the coty- 
ledons of Phalaris were to light, we endeavoured to 
trace their circumnutation in darkness by the aid ( f 
a small wax taper, held for a minute or two at each 
observation in nearly the same position, a little on the 
left side in front of the vertical glass on which the 
tracing was made. The seedlings were thus observed 
seventeen times in the course of the day, at intervals of 
from half to three-quarters of an hour ; and late in the 
evening we were surprised to find that all the 29 coty- 
ledons were greatly curved and pointed towards the 
vertical glass, a little to the left where the taper had 
been held. The tracings showed that they had tra- 
velled in zigzag lines. Thus, an exposure to a feeble 
light for a very short time at the above specified 
intervals, sufficed to induce well-marked heliotropism. 
An analogous case was observed with the hypocotyls 
of Solanum lycopersicum. We at first attributed this 
result to the after-effects of the light on each occasion ; 
but since reading Wiesner's observations,* which will 
be referred to in the last chapter, we cannot doubt that 
an intermittent light is more efficacious than a con- 
tinuous one, as plants are especially sensitive to any 
contrast in its amount. 

The cotyledons of Phalaris bend much more slowly 
towards a very obscure light than towards a bright 
one. Thus, in the experiments with seedlings placed 
in a dark room at 12 feet from a very small lamp, they 
were just perceptibly and doubtfully curved towards it 
after 3 h., and only slightly, yet certainly, after 4 h 

* l Sitz. der k. Akad. der Wis sen sell.* (Vienna), Jan. 1880, p. 12. 



i58 SENSITIVENESS TO LIGHT. Chap. IX 

After 8 h. 40 m. the chords of their arcs were deflected 
from the perpendicular by an average angle of only 
16°. Had the light been bright, they would have 
become much more curved in between 1 and 2 h. 
Several trials were made with seedlings placed at 
various distances from a small lamp in a dark room ; 
but we will give only one trial. Six pots were placed 
at distances of 2, 4, 8, 12, 16, and 20 feet from the 
lamp, before which they were left for 4 h. As light 
decreases in a geometrical ratio, the seedlings in the 
2nd pot received |th, those in the 3rd pot -iVth, 
those in the 4th gVth, those in the 5th gVth, and those 
in the 6th T^-o-th of the light received by the seedlings in 
the first or nearest pot. Therefore it might have been 
expected that there would have been an immense differ- 
ence in the degree of their heliotropic curvature in the 
several pots ; and there was a well-marked difference 
between those which stood nearest and furthest from 
the lamp, but the difference in each successive pair of 
pots was extremely small. In order to avoid prejudice, 
we asked three persons, who knew nothing about the 
experiment, to arrange the pots in order according to 
the degree of curvature of the cotyledons. The first 
jDerson arranged them in proper order, but doubted 
long between the 12 feet and 16 feet pots ; yet these 
two received light in the proportion of 36 to 64. The 
second person also arranged them properly, but 
doubted between the 8 feet and 12 feet pots, which 
received light in the proportion of 16 to 36. The 
third person arranged them in wrong order, and 
doubted about four of the pots. This evidence shows 
conclusively how little the curvature of the seedlings 
differed in the successive pots, in comparison with the 
great difference in the amount of light which they 
received ; and it should be noted that there was no 



Chap. IX. SENSITIVENESS TO LIGHT. 459 

excess of superfluous light, for the cotyledons became 
but little and slowly curved even in the nearest pot. 
Close to the 6th pot, at the distance of 20 feet from 
the lamp, the light allowed us just to distinguish 
a dot 3*56 mm. (*14 inch) in diameter, made with 
Jndian ink on white paper, but not a dot 2*29 mm. 
(•09 inch) in diameter. 

The degree of curvature of the cotyledons of Phalaris 
within a given time, depends not merely on the 
amount of lateral light which they may then receive, 
but on that which they have previously received from 
above and on all sides. Analogous facts have been 
given with respect to the nyctitropic and periodic 
movements of plants. Of two pots containing seedlings 
of Phalaris which had germinated in darkness, one was 
still kept in the dark, and the other was exposed (Sept. 
26th) to the light in a greenhouse during a cloudy day 
and on the following bright morning. On this morn- 
ing (27th), at 10.30 a.m., both pots were placed in a 
box, blackened within and open in front, before a 
north-east window, protected by a linen and muslin 
blind and by a towel, so that but little light was 
admitted, though the sky was bright. Whenever the 
pots were looked at, this was done as quickly as pos- 
sible, and the cotyledons were then held transversely 
with respect to the light, so that their curvature could 
not have been thus increased or diminished. After 
50 m. the seedlings which had previously been kept 
in darkness, were perhaps, and after 70 m. were cer- 
tainly, curved, though very slightly, towards the 
window. After 85 m. some of the seedlings, which 
had previously been illuminated, were perhaps a little 
affected, and after 100 m. some of the younger ones 
were certainly a little curved towards the light. At 
this time (i.e. after 100 m.) there was a plain difference 



160 SENSITIVENESS TO LIGHT. Chap. IX 

in the curvature of the seedlings in the two pots. 
After 2 h. 12 m. the chords of the arcs of four of 
the most strongly curved seedlings in each pot were 
measured, and the mean angle from the perpendicular 
of those which had previously been kept in darkness 
was 19°, and of those which had previously been illu- 
minated only 7°. Nor did this difference diminish 
during two additional hours. As a check, the seed- 
lings in both pots were then placed in complete dark- 
ness for two hours, in order that apogeotropism should 
act on them ; and those in the one pot which were 
little curved became in this time almost completely 
upright, whilst the more curved ones in the other pot 
still remained plainly curved. 

Two days afterwards the experiment was repeated, 
with the sole difference that even less light was 
admitted through the window, as it was protected by a 
linen and muslin blind and by two towels ; the sky, 
moreover, was somewhat less bright. The result was 
the same as before, excepting that everything occurred 
rather slower. The seedlings which had been pre- 
viously kept in darkness were not in the least curved 
after 54 m., but were so after 70 m. Those which had 
previously been illuminated were not at all affected 
until 130 m. had elapsed, and then only slightly. 
After 145 m. some of the seedlings in this latter pot 
were certainly curved towards the light ; and there 
was now a plain difference between the two pots. After 
3 h. 45 m. the chords of the arcs of 3 seedlings in 
each pot were measured, and the mean angle from the 
perpendicular was 16° for those in the pot which had 
previously been kept in darkness, and only 5° lbi 
those which had previously been illuminated. 

The curvature of the cotyledons of Phalaris towards 
a lateral light is therefore certainly influenced by the 



Chap. IX. SENSITIVENESS TO LIGHT. 40 J 

degree to which they have been previously illu- 
minated. We shall presently see that the influence 
of light on their bending continues for a short time 
after the light has been extinguished. These facts, as 
well as that of the curvature not increasing or de- 
creasing in nearly the same ratio with that of the 
amount of light which they receive, as shown in the 
trials with the plants before the lamp, all indicate 
that light acts on them as a stimulus, in somewhat 
the same manner as on the nervous system of animals, 
and not in a direct manner on the cells or cell-walls 
which by their contraction or expansion cause the 
curvature. 

It has already been incidentally shown how slowly 
the cotyledons of Phalaris bend towards a very dim 
light ; but when they were placed before a bright 
paraffin lamp their tips were all curved rectangularly 
towards it in 2 h. 20 m. The hypocotyls of Solanum 
ly coper sicum had bent in the morning at right angles 
towards a north-east window. At 1 p.m. (Oct. 21st) the 
pot was turned round, so that the seedlings now pointed* 
from the light, but by 5 p.m. they had reversed their 
curvature and again pointed to the light. They had 
thus passed through 180° in 4 h., having in the 
morning previously passed through about 90°. But the 
reversal of the first half of the curvature will have 
been aided by apogeotropism. Similar cases were 
observed with other seedlings, for instance, with those 
of Sinapis alba. 

We attempted to ascertain in how short a time 
light acted on the cotyledons of Phalaris, but this 
was difficult on account of their rapid circumnutating 
movement; moreover, they differ much in sensibility, 
fcceording to age; nevertheless, some of our observa- 
tions are worth giving. Pots with seedlings we e 



i62 SENSITIVENESS TO LIGHT. Chap. IX. 

placed under a microscope provided with an eye-piece 
micrometer, of which each division equalled -g-J-oth of an 
inch (0'051 mm.) ; and they were at first illuminated 
by light from a paraffin lamp passing through a solu- 
tion of bichromate of potassium, which does not induce 
heliotropism. Thus the direction in which the coty- 
ledons were circumnutating could be observed inde- 
pendently of any action from the light ; and they could 
be made, by turning round the pots, to circumnutate 
transversely to the line in which the light would strike 
them, as soon as the solution was removed. The fact 
that the direction of the circumnutating movement 
might change at any moment, and thus the plant 
might bend either towards or from the lamp indepen- 
dently of the action of the light, gave an element of 
uncertainty to the results. After the solution had 
been removed, five seedlings which were circumnutat- 
ing transversely to the line of light, began to move 
towards it, in 6, 4, 7J, 6, and 9 minutes. In one of 
these cases, the apex of the cotyledon crossed five 
of the divisions of the micrometer (i.e. j^oth of an 
inch, or 0*254 mm.) towards the light in 3 m. Of two 
seedlings which were moving directly from the light at 
the time when the solution was removed, one began to 
move towards it in 13 m., and the other in 15 m. 
This latter seedling was observed for more than an 
hour and continued to move towards the light ; it 
crossed at one time 5. divisions of the micrometer 
(0*254 mm.) in 2 m. 30 s. In all these cases, the 
movement towards the light was extremely unequal in 
rate, and the cotyledons often remained almost sta- 
tionary for some minutes, and two of them retrograded 
a little. Another seedling which was circumnutating 
transversely to the line of light, moved towards it in 
4 m. after the solution was removed ; it then remained 



Cbai\ IX. SENSITIVENESS TO LIGHT. 468 

almost stationary for 10 m. ; then crossed 5 divisions 
of the micrometer in 6 m.; and then 8 divisions in 
11 m. This unequal rate of movement, interrupted 
by pauses, and at first with occasional retrogressions, 
accords well with our conclusion that heliotropism 
consists of modified circumnutation. 

In order to observe how long the after-effects of 
light lasted, a pot with seedlings of Phalaris, which 
had germinated in darkness, was placed at 10.40 a.m. 
before a north-east window, being protected on all 
other sides from the light; and the movement of a 
cotyledon was traced on a horizontal glass. It cir- 
cumnutated about the same space for the first 24 m., 
and during the next 1 h. 33 m. moved rapidly towards 
the light. The light was now (i.e. after 1 h. 57 m.) 
completely excluded, but the cotyledon continued 
bending in the same direction as before, certainly for 
more than 15 m., probably for about 27 m. The doubt 
arose from the necessity of not looking at the seed- 
lings often, and thus exposing them, though momen- 
tarily, to the light. This same seedling was now kept 
in the dark, until 2.18 p.m., by which time it had 
reacquired through apogeotropism its original upright 
position, when it was again exposed to the light from 
a clouded sky. By 3 p.m. it had moved a very short 
distance towards the light, but during the next 45 in. 
travelled quickly towards it. After this exposure of 
1 h. 27 m. to a rather dull sky, the light was again 
completely excluded, but the cotyledon continued to 
bend in the same direction as before for 14 m. within 
a very small limit of error. It was then placed in 
the dark, and it now moved backwards, so that after 
1 h. 7 m. it stood close to where it had started from at 
2.18 p.m. These observations show that the coty- 
ledons of Phalaris, after being exposed to a lateral 



464 SENSI1IVENESS TO LIGHT. Chap. IX. 

light, continue to bend in the same direction for 
between a quarter and half an hour. 

In the two experiments just given, the cotyledons 
moved backwards or from the window shortly after 
being subjected to darkness ; and whilst tracing the 
circumnutation of various kinds of seedlings exposed 
to a lateral light, we repeatedly observed that late in 
the evening, as the light waned, they moved from it. 
This fact is shown in some of the diagrams given in 
the last chapter. We wished therefore to learn whether 
this was wholly due to apogeotropism, or whether an 
organ after bending towards the light tended from 
any other cause to bend from it, as soon as the light 
failed. Accordingly, two pots of seedling Phalaris 
and one pot of seedling Brassica were exposed for 8 h. 
before a paraffin lamp, by which time the cotyledons 
of the former and the hypocotyls of the latter were bent 
rectangularly towards the light. The pots were now 
quickly laid horizontally, so that the upper parts of 
the cotyledons and of the hypocotyls of 9 seedlings 
projected vertically upwards, as proved by a plumb-line. 
In this position they could not be acted on by apo- 
geotropism, and if they possessed any tendency to 
straighten themselves or to bend in opposition to their 
former heliotropic curvature, this would be exhibited, 
for it would be opposed at first very slightly by apogeo- 
tropism. They were kept in the dark for 4 h., during 
which time they were twice looked at ; but no uniform 
bending in opposition to their former heliotropic 
curvature could be detected. We have said uniform 
bending, because they circumnutated in their new 
position, and after 2 h. were inclined in different 
directions (between 4° and 11°) from the perpendicular. 
Their directions were also changed after two additions] 
hours, and again on the following morning. We may 



Chap. IX. SENSITIVENESS TO LIGHT. 4H5 

therefore conclude that the bending back of plants 
from a light, when this becomes obscure or is extin- 
guished, is wholly due to apogeotropism.* 

In our various experiments we were often struck 
with the accuracy with which seedlings pointed to a 
light although of small size. To test this, many seed- 
lings of Phalaris, which had germinated in darkness in 
a very narrow box several feet in length, were placed 
in a darkened room near to and in front of a lamp 
having a small cylindrical wick. The cotyledons at 
the two ends and in the central part of the box, would 
therefore have to bend in widely different directions 
in order to point to the light. After they had become 
rectangularly bent, a long white thread was stretched 
by two persons, close over and parallel, first to one and 
then to another cotyledon ; and the thread was found 
in almost every case actually to intersect the small 
circular wick of the now extinguished lamp. The 
deviation from accuracy never exceeded, as far as we 
could judge, a degree or two. This extreme accuracy 
seems at first surprising, but is not really so, for an 
upright cylindrical stem, whatever its position may 
be with respect to the light, would have exactly half 
its circumference illuminated and half in shadow ; and 
as the difference in illumination of the two sides is 
the exciting cause of heliotropism, a cylinder would 
naturally bend with much accuracy towards the light. 
The cotyledons, however, of Phalaris are not cylin- 
drical, but oval in section; and the longer axis was 
to the shorter axis (in the one which was measured) 
as 100 to 70. Nevertheless, no difference could be 



* It appears from a reference heliotropically is at the same time 

in Wiesner (' Die Undulirende striving, through apogeotropism, 

Nutation der Internodien,' p. 7), to raise itself into a vertical posi- 

that H. Mailer of Thurgau found tion. 
that a stem which is bending 



Itff) SENSITIVENESS TO LIGHT. Chap. IX 

detected in the accuracy of their bending, whether 
they stood with their broad or narrow sides facing 
the light, or in any intermediate position; and so it 
was with the cotyledons 01 Avena sativa, which are 
likewise oval in section. Now, a little reflection will 
show that in whatever position the cotyledons may 
stand, there will be a line of greatest illumination, 
exactly fronting the light, and on each side of this 
line an equal amount of light will be received ; but 
if the oval stands obliquely with respect to the light, 
this will be diffused over a wider surface on one side 
of the central line than on the other. We may there- 
fore infer that the same amount of light, whether 
diffused over a wider surface or concentrated on a 
smaller surface, produces exactly the same effect ; for 
the cotyledons in the long narrow box stood in all 
sorts of positions with reference to the light, yet all 
pointed truly towards it. 

That the bending of the cotyledons to the light 
depends on the illumination of one whole side or on 
the obscuration of the whole opposite side, and not on 
a narrow longitudinal zone in the line of the light 
being affected, was shown by the effects of painting 
longitudinally with Indian ink one side of five coty- 
ledons of Phalaris. These were then placed on a table 
near to a south-west window, and the painted half was 
directed either to the right or left. The result was that 
instead of bending in a direct line towards the window, 
they were deflected from the window and towards the 
unpainted side, by the following angles, 35°, 83°, 31°, 
43 3 , and 39°. It should be remarked that it was hardly 
possible to paint one-half accurately, or to place all 
the seedlings which are oval in section in quite the 
same position relatively to the light; and this will 
account for the differences in the angles. Five coty« 



Chap. IX. SENSITIVENESS TO LIGHT. 4(57 

ledons of Avena were also painted in the same manner, 
but with greater care; and they were laterally de- 
flected from the line of the window, towards the 
unpainted side, by the following angles, 44°, 44°, 55°, 
51°, and 57°. This deflection of the cotyledons from 
the window is intelligible, for the whole unpainted 
side must have received some light, whereas the oppo- 
site and painted side received none ; but a narrow 
zone on the unpainted side directly in front of the 
window will have received most light, and all the 
hinder parts (half an oval in section) less and less light 
in varying degrees ; and we may conclude that the 
angle of deflection is the resultant of the action of the 
light over the whole of the unpainted side. 

It should have been premised that painting with 
Indian ink does not injure plants, at least within 
several hours ; and it could injure them only by stop- 
ping respiration. To ascertain whether injury was thus 
soon caused, the upper halves of 8 cotyledons of Avena 
were thickly coated with transparent matter, — 4 with 
gum, and 4 with gelatine ; they were placed in the 
morning before a window, and by the evening they 
were normally bowed towards the light, although the 
coatings now consisted of dry crusts of gum and 
gelatine. Moreover, if the seedlings which were painted 
longitudinally with Indian ink had been injured on 
the painted side, the opposite side would have gone 
on growing, and they would consequently have become 
bowed towards the painted side ; whereas the curvature 
was always, as we have seen, in the opposite direction, 
or towards the unpainted side which was exposed to 
the light. We witnessed the effects of injuring longi- 
tudinally one side of the cotyledons of Avena and 
Phalaris ; for before we knew that grease was highly 
injurious to them, several were painted down one side 



£68 TRANSMITTED EFFECTS OF LIGHT. Chap. IX 

with a mixture of oil and larnp-black, and were then 
exposed before a window ; others similarly treated were 
afterwards tried in darkness. These cotyledons soon 
became plainly bowed towards the blackened side, 
evidently owing to the grease on this side having 
checked their growth, whilst growth continued on the 
opposite side. But it deserves notice that the curva- 
ture differed from that caused by light, which ulti- 
mately becomes abrupt near the ground. These 
seedlings did not afterwards die, but were much injured 
and grew badly. 

Localised Sensitiveness to Light, and its 
transmitted effects. 

Phalaris Canariensis. — Whilst observing the accu- 
racy with which the cotyledons of this plant became 
bent towards the light of a small lamp, we were 
impressed with the idea that the uppermost part deter- 
mined the direction of the curvature of the lower part. 
When the cotyledons are exposed to a lateral light, 
the upper part bends first, and afterwards the bending 
gradually extends down to the base, and, as we shall 
presently see, even a little beneath the ground. 
This holds good with cotyledons from less than 
*1 inch (one was observed to act in this manner which 
was only *03 in height) to about '5 of an inch in 
height ; but when they have grown to nearly an inch 
in height, the basal part, for a length of '15 to *2 of 
an inch above the ground, ceases to bend. As with 
young cotyledons the lower part goes on bending, 
after the upper part has become well arched towards 
a lateral light, the apex would ultimately point to 
the ground instead of to the light, did not the upper 
part reverse its curvature and straighten itself, as 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 469 

soon as the upper convex surface of the bowed- 
down portion received more light than the lower 
concave surface. The position ultimately assumed by 
young and upright cotyledons, exposed to light enter- 
ing obliquely from above through a window, is shown 
in the accompanying figure (Fig. 181) ; and here it 
may be seen that the whole upper part has become 
very nearly straight. When the cotyledons were 
exposed before a bright lamp, standing on the same 
level with them, the upper part, which was at first 

Fie. 181. 






Phalaris Canariensis : cotyledons after exposure in a box open on one side 
in front of a south-west window during 8 h. Curvature towards the 
light accurately traced. The short horizontal lines show the level ol 
the ground. 

great] y arched towards the light, became straight and 
strictly parallel with the surface of the soil in the 
pots; the basal part being now rectangularly bent. 
All this great amount of curvature, together with the 
subsequent straightening of the upper part, was often 
effected in a few hours. 

After the uppermost part has become bowed a little to the 
light, its overhanging weight must tend to increase the curva- 
ture of the lower part; but any such effect was shown in several 
ways to be quite insignificant. When little caps of tin-foil 
(hereafter to be described) were placed on the summits of the 
cotyledons, though this must have added considerably to their 
weight, the rate or amount of bending was not thus increased. 
But the best evidence was afforded by placing pots with seedlings 
of Phalaris before a lamp in such a position, that the cotyledons 
were horizontally extended and projected at right angles to the 
line of light. . In the course of 5§ h. they were directed towards 
the light with their bases bent at right angles ; ai i this abrupt 



470 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

curvature could uot have been aided in the least by the weight 
cf the upper part, which acted at right angles to the plane of 
curvature. 

It will be shown that when the upper halves of the coty- 
ledons of Phalaris and Avena were enclosed in little pipes of 
tin-foil or of blackened glass, in which case the upper part was 
mechanically prevented from bending, the lower and unenclosed 
part did not bend when exposed to a lateral light; and it 
occurred to us that this fact might be due, not to the exclusion 
of the light from the upper part, but to some necessity of the 
bending gradually travelling down the cotyledons, so that 
unless the upper part first became bent, the lower could not 
bend, however much it might be stimulated. It was necessary 
for our purpose to ascertain whether this notion was true, and it 
was proved false; for the lower halves of several cotyledons 
became bowed to the light, although their upper halves were 
enclosed in little glass tubes (not blackened), which prevented, 
as far as we could judge, their bending. Nevertheless, as the 
part within the tube might possibly bend a very little, fine rigid 
rods or flat splinters of thin glass were cemented with shellac to 
one side of the upper part of 15 cotyledons; and in six cases 
they were in addition tied on with threads. They were thus 
forced to remain quite straight. The resnlt was that the lower 
halves of all became bowed to the light, but generally not in so 
great a degree as the corresponding part of the free seedlings 
in the same pots ; and this may perhaps be accounted for by 
some slight degree of injury having been caused by a consider- 
able surface having been smeared with shellac. It may be 
added, that when the cotyledons of Phalaris and Avena are 
acted on by apogeotropism, it is the upper part which begins 
first to bend; and when this part was rendered rigid in the 
manner just described, the upward curvature of the basal part 
was not thus prevented. 

To test our belief that the upper part of the cotyledons of 
Phalaris, when exposed to a lateral light, regulates the bending 
of the lower part, many experiments were tried ; but most of our 
first attempts proved useless from various causes not worth 
specifying. Seven cotyledons had their tips cut off for lengths 
varying between *1 and T6 of an inch, and these, when left 
exposed all day to a lateral light, remained upright. In another 
set of 7 cotyledons, the tips were cut off for a length of only 
about *05 of an inch (1-27 mm.) and these became bowed towards 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 471 

a lateral light, but not nearly so much as the many other seed- 
lings in the same pots. This latter case shows that cutting off 
the tips does not by itself injure the plants so seriously as to 
prevent heliotropism ; but we thought at the time, that such 
injury might follow when a greater length was cut off, as in the 
tirst set of experiments. Therefore, no more trials of this kind 
were made, which we now regret ; as we afterwards found that 
when the tips of three cotyledons were cut off for a length of 
2 inch, and of four others for lengths of *14, *12, T, and *07 
inch, and they were extended horizontally, the amputation did 
not interfere in the least with their bending vertically upwards, 
through the action oi apogeotropism, like unmutilated speci- 
mens. It is therefore extremely improbable that the amputation 
of the tips for lengths of from T to TI inch, could from the 
injury thus caused have prevented the lower part from bending 
towards the light. 

We next tried the effects of covering the upper part of the 
cotyledons of Phalaris with little caps which were impermeable 
to light ; the whole lower part being left fully exposed before a 
south-west window or a bright paraffin lamp. Some of the caps 
were made of extremely thin tin-foil blackened within; these 
had the disadvantage of occasionally, though rarely, being too 
heavy, especially when twice folded. The basal edges could be 
pressed into close contact with the cotyledons ; though this 
again required care to prevent injuring them. Nevertheless, 
any injury thus caused could be detected by removing the caps, 
and trying whether the cotyledons were then sensitive to light. 
Other caps were made of tubes of the thinnest glass, which 
when painted black served well, with the one great disadvantage 
that the lower ends could not be closed- But tubes were used 
which fitted the cotyledons almost closely, and black paper was 
placed on the soil round each, to check the upward reflection of 
light from the soil. Such tubes were in one respect far better 
than caps of tin-foil, as it was possible to cover at the same 
time some cotyledons with transparent and others with opaque 
tubes ; and thus our experiments could be controlled. It should 
be kept in mind that young cotyledons were selected for trial, 
and that these when not interfered with become bowed down 
to the ground towards the light. 

We will begin with the glass-tubes. The summits of nine 
sotyledons, differing somewhat in height, wero enclosed for 
rather less than half their lengths in uncoloured or transparent 
31 



472 TEANSMITTED EFFECTS OF LIGHT. Chav. IX 

tubes ; and these were then exposed before a south-west window 
on a bright day for 8 h. All of them became strongly curved 
towards the light, in the same degree as the many other free 
seedlings in the same pots ; so that the glass-tubes certainly did 
not prevent the cotyledons from bending towards the light. 
Nineteen other cotyledons were, at the same time, similarly 
enclosed in tubes thickly painted with Indian ink. On five of 
them, the paint, to our surprise, contracted after exposure 
to the sunlight, and very narrow cracks were formed, through 
which a little light entered ; and these five cases were rejected. 
Of the remaining 14 cotyledons, the lower halves of which had 
been fully exposed to the light for the whole time, 7 continued 
quite straight and upright ; 1 was considerably bowed to the 
light, and 6 were slightly bowed, but with the exposed bases of 
most of them almost or quite straight. It is possible that some 
light may have been reflected upwards from the soil and entered 
the bases of these 7 tubes, as the sun shone brightly, though 
bits of blackened paper had been placed on the soil round 
them. Nevertheless, the 7 cotyledons which were slightly 
bowed, together with the 7 upright ones, presented a most re- 
markable contrast in appearance with the maoy other seedlings 
in the same pots to which nothing had been done. The 
blackened tubes were then removed from 10 of these seedlings, 
and they were now exposed before a lamp for 8 h. : 9 of them 
became greatly, and 1 moderately, curved towards the light, 
proving that the previous absence of any curvature in the 
basal part, or the presence of only a slight degree of curvature 
there, was due to the exclusion of light from the upper part. 

Similar observations were made on 12 younger cotyledons 
with their upper halves enclosed within glass-tubes coated with 
black varnish, and with their lower halves fully exposed to 
bright sunshine. In these younger seedlings the sensitive zone 
seems to extend rather lower down, as was observed on some 
other occasions, for two became almost as much curved towards 
the light as the free seedlings; and the remaining ten were 
slightly curved, although the basal part of several of them, 
which normally becomes more curved than any other part, 
exhibited hardly a trace of curvature. These 12 seedlings 
taken together differed greatly in their degree of curvature from 
all the many other seedlings in the same pots. 

Better evidence of the efficiency of the blackened tubes was 
incidentally afforded by some experiments hereafter to be given, 



Chap. IX TKANSMITTED EFFECTS OF LIGHT. 478 

in which the upper halves of 14 cotyledons were enclosed ir 
tubes from which an extremely narrow stripe of the black 
varnish had been scraped off. These cleared stripes were 
not directed towards the window, but obliquely to one side 
of the room, so that only a very little light could act on the 
upper halves of the cotyledons. These 14 seedlings remained 
during eight hours of exposure before a south-west window on 
a hazy day quite upright; whereas all the other many free 
seedlings in the same pots became greatly bowed towards the 
light. 

We will now turn to the trials with caps made of very thin 
tin-foil. These were placed at different times on the summits of 
24 cotyledons, and they extended down for a length of between 
T5 and 2 of an inch. The seedlings were exposed to a lateral 
light for periods varying between 6 h. cSO m. and 7 h. 45 m., 
which sufficed to cause all the other seedlings in the same pots 
to become almost rectangularly bent towards the light. They 
varied in height from only "04 to 1T5 inch, but the greater 
number were about 75 inch. Oi" the 24 cotyledons with their 
summits thus protected, 3 became much bent, but not in the 
direction of the light, and as they did not s-traighten themselves 
through apogeotropism during the following night, either the 
caps were too heavy or the plants themselves were in a weak 
condition; and these three cases may be excluded. There 
are left for consideration 21 cotyledons ; of these 17 remained 
all the time quite upright ; the other 4 became slightly inclined 
to the light, but not in a degree comparable with that of the 
many free seedlings in the same pots. As the glass-tubes, when 
unpainted, did not prevent the cotyledons from becoming 
greatly bowed, it cannot be supposed that the caps o f very 
thin tin-foil did so, except through the exclusion of the light. 
To prove that the plants had not been injured, the caps were 
removed from 6 of the upright seedlings, and these were exposed 
before a paraffin lamp for the same length of time as before 
and they now all became greatly curved towards the light. 

As caps between '15 and '2 of an inch in depth were thus 
proved to be highly efficient in preventing the cotyledons from 
bending towards the light, 8 other cotyledons were protected 
with caps between only 06 and T2 in depth. Of these, two 
remained vertical, one was considerably and five slightly curved 
towards the light, but far less so than the free seedlings in the 
same pots. 



i74 TKANSMITTED EFFECTS OF LIGHT. Chap. IX 

Another trial was made in a clLTerent manner, namely, It? 
handaging with strips of tin-foil, about *2 in breadth, the npper 
part, but not the actual summit, of. eight moderately young 
seedlings a little over half an inch in height. The summits and 
the basal parts were thus left fully exposed to a lateral light 
during 8 h. ; an upper intermediate zone being protected. 
With four of these seedlings the summits were exposed for 
a length of '05 inch, and in two of them this part became 
curved towards the light, but the whole lower part remained 
quite upright; whereas the entire length of the other two 
seedlings became slightly curved towards the light. The 
summits of the four other seedlings were exposed for a length 
of '04 inch, and of these one remained almost upright, whilst 
the other three became considerably curved towards the light. 
The many free seedlings in the same pots were all greatly 
curved towards the light. 

From these several sets of experiments, including those with 
the glass-tubes, and those when the tips were cut off, we may 
infer that the exclusion of light from the upper part of the 
cotyledons of Phalaris prevents the lower part, though fully 
exposed to a lateral light, from becoming curved. The summit 
for a length of "04 or "05 of an inch, though it is itself sensitive 
and curves towards the light, has only a slight power of causing 
the lower part to bend. Nor has the exclusion of light from the 
summit for a length of *1 of an inch a strong influence on the 
curvature of the lower part. On the other hand, an exclusion 
for a length of between *15 and "2 of an inch, or of the whole 
upper half, plainly prevents the lower and fully illuminated 
part from becoming curved in the manner (see Fig. 181) which 
invariably occurs when a free cotyledon is exposed to a lateral 
light. With very young seedlings the sensitive zone seems to 
extend rather lower down relatively to their height than in older 
seedlings. We must therefore conclude that when seedlings 
are freely exposed to a lateral light some influence is trans- 
mitted from the upper to the lower part, causing the latter to 
bend. 

This conclusion is supported by what may be seen to occur 
on a small scale, especially with young cotyledons, without any 
artificial exclusion of the light; for they bend beneath the earth 
where no light can enter. Seeds of Phalaris were covered 
with a layer one-fourth of an inch in thickness of very fine 
sand, consisting of extremely minute grains of silex coated with 



Chap. IX. TKANSMITTED EFFECTS OF LIGHT. 475 

oxide of iron. A layer of this sand, moistened to the same 
degree as that over the seeds, was spread over a glass-plate ; and 
when the layer was '05 of an inch in thickness (carefully mea- 
sured) no light from a bright sky could be seen to pass through 
it, unless it was viewed through a ]ong blackened tube, and 
then a trace of light could be detected, but probably much too 
little to affect any plant. A layer *1 of an inch in thickness was 
quite impermeable to light, as judged by the eye aided by the tube. 
It may be worth adding that the layer, when dried, remained 
equally impermeable to light. This sand yielded to very slight 
pressure whilst kept moist, and in this state did not contract 
or crack in the least. In a first trial, cotyledons which had 
grown to a moderate height were exposed for 8 h. before a paraffin 
lamp, and they became greatly bowed. At their bases on the 
shaded side opposite to the light, well-defined, crescentic, open 
furrows were formed, which (measured under a microscope with 
a micrometer) were from 02 to 03 of an inch in breadth, and 
these had evidently been left by the bending of the buried bases 
of the cotyledons towards the light. On the side of the light 
the cotyledons were in close contact with the sand, which was a 
very little heaped up. By removing with a sharp knife the 
sand on one side of the cotyledons in the line of the light, the 
bent portion and the open furrows were found to extend down 
to a depth of about -1 of an inch, where no light could enter. 
The chords of the short buried arcs formed in four cases angles 
of 11°, 13 3 , 15°, and 18°, with the perpendicular. By the 
following morning these short bowed portions had straightened 
themselves through apogeotropism. 

In the next trial much younger cotyledons were similarly 
treated, but were exposed to a rather obscure lateral light. 
After some hours, a bowed cotyledon, - 3 inch in height, had an 
open furrow on the shaded side ■ 04 inch in breadth ; another 
cotyledon, only *13 inch in height, had left a furrow *02 inch in 
breadth. But the most curious case was that of a cotyledon which 
had just protruded above the ground and was only ' 03 inch in 
height, and this was found to be bowed in the direction of the 
light to a depth of "2 of an inch beneath the surface. From 
what we know of the impermeability of this sand to light, the 
upper illuminated part in these several cases must have deter- 
mined the curvature of the lower buried portions. But an 
apparent cause of doubt may be suggested : as the cotyledons 
are continually circumnutating, they tend to form a minut€ 



§76 TRANSMITTED EFFECTS OF LIGHT. Chak IX 

crack or furrow all round their bases, which would admit a 
little light on all sides; but this would not happen when thej 
were illuminated laterally, for we know that they quickly bend 
towards a lateral light, and they then press so firmly against the 
sand on the illuminated side as to furrow it, and this would 
effectually exclude light on this side. Any light admitted on 
the opposite and shaded side, where an open furrow is formed, 
would tend to counteract the curvature towards the lamp or 
other source of the light. It may be added, that the use of fine 
moist sand, which yields easily to pressure, was indispensable 
in the above experiments ; for seedlings raised in common soil, 
not kept especially damp, and exposed for 9 h. 30 m. to a strong 
lateral light, did not form an open furrow at their bases on the 
shaded side, and were not bowed beneath the surface. 

Perhaps the most striking proof of the action of the upper 
on the lower part of the cotyledons of Phalaris, when laterally 
illuminated, was afforded by the blackened glass-tubes (before 
alluded to) with very narrow stripes of the varnish scraped 
off on one side, through which a little light was admitted. 
The breadth of these stripes or slits varied between *01 and 
•02 inch (*25 and *51 mm.). Cotyledons with their upper 
halves enclosed in such tubes were placed before a south-west 
window, in such a position, that the scraped stripes did not 
directly face the window, but obliquely to one side. The seed- 
lings were left exposed for 8 h., before the close of which time 
the many free seedlings in the same pots had become greatly 
bowed towards the window. Under these circumstances, the 
whole lower halves of the cotyledons, which had their summits 
enclosed in the tubes, were fully exposed to the light of the 
sky, whilst their upper halves received exclusively or chiefly 
diffused light from the room, and this only through a very 
narrow slit on one side. Now, if the curvature of the lower 
part had been determined by the illumination of this part, all 
the cotyledons assuredly would have become curved towards 
the window; but this was far from being the case. Tubes 
of the kind just described were placed on several occasions 
over the upper halves of 27 cotyledons ; 14 of them remained 
all the time quite vertical; so that sufficient diffused light 
did not enter through the narrow slits to produce any effect 
whatever; and they behaved in the same manner as if their 
upper halves had been enclosed in completely blackened tubes. 
The lower halves of the 13 other cotvledons became bowed 



Dhap. IX. TRANSMITTED EFFECTS OF LIGHT. 477 

not directly in the line of the window, but obliquely towards 
it ; one pointed at an angle of only 18°, but the remaining 12 
at angles varying between 45° and 62° from the line of the 
window. At the commencement of the experiment, pins had 
been laid on the earth in the direction towards which the slits in 
the varnish faced ; and in this direction alone a small amount 
of diffused light entered. At the close of the experiment, 7 ot 
the bowed cotyledons pointed exactly in the line of the pins, 
and 6 of them iu a line between that of the pins and that of the 
window. This intermediate position is intelligible, for any light 
from the sky which entered obliquely through the slits would 
be much more efficient than the diffused light which entered 
directly through them. After the 8 h. exposure, the contrast 
in appearance between these 13 cotyledons and the many other 
seedlings in the same pots, which were all (excepting the above 
14 vertical ones) greatly bowed in straight and parallel lines 
towards the window, was extremely remarkable. It is therefore 
certain that a little weak light striking the upper halves of the 
cotyledons of Phalaris, is far more potent in determining the 
direction of the curvature of the lower halves, than the full 
illumination of the latter during the whole time of exposure. 

In confirmation of the above results, the effect of thickly 
painting with Indian ink one side of the upper part of three coty- 
ledons of Phalaris, for a length of ■ 2 inch from their tips, may be 
worth giving. These were placed so that the unpainted surface 
was directed not towards the window, but a little to one side ; 
and they all became bent towards the unpainted side, and from 
the line of the window by angles amounting to 31°, 35°, and 83°. 
The curvature in this direction extended down to their bases, 
although the whole lower part was fully exposed to the light 
from the window. 

Finally, although there can be no doubt that the illumination 
of the upper part of the cotyledons of Phalaris greatly affects 
the power and manner of bending of the lower part, jet some 
observations seemed to render it probable that the simultaneous 
stimulation of the lower part by light greatly favours, or is 
almost necessary, for its well-marked curvature ; but our experi- 
ments were not conclusive, owing to the difficulty of excluding 
light from the lower halves without mechanically preventing 
their curvature. 

Avena sativa. — The cotyledons of this plant become quickly 
oo wed towards a lateral light, exactly like those of Phalaris. 



1:78 TRANSMITTED EFFECTS OF LIGHT. Chap. IX 

Experiments similar to the foregoing ones were tried, and \vc 
will give the results as briefly as possible. They are somewhat 
less conclusive than in the case of Phalaris, and this may 
possibly be accounted for by the sensitive zone varying in exten- 
sion, in a species so long cultivated and variable as the common 
Oat. Cotyledons a little under three-quarters of an inch in 
height were selected for trial : six had their summits protected 
from light by tin-foil caps, *25 inch in depth, and two others by 
caps *3 inch in depth. Of these 8 cotyledons, five remained 
upright during 8 hours of exposure, although their lower parts 
were fully exposed to the light all the time; two were very slightly, 
and one considerably, bowed towards it. Caps only • 2 or • 22 inch 
in depth were placed over 4 other cotyledons, and now only one 
remained upright, one was slightly, and two considerably bowed 
to the light. In this and the following cases all the free seedlings 
in the same pots became greatly bowed to the light. 

Our next trial was made with short lengths of thin and 
fairly transparent quills; for glass-tubes of sufficient diameter 
to go over the cotyledons would have been too heavy. Firstly, 
the summits of 13 cotyledons were enclosed in unpainted 
quills, and of these 11 became greatly and 2 slightly bowed 
to the light ; so that the mere act of enclosure did not prevent 
the lower part from becoming bowed. Secondly, the summits 
of 11 cotyledons were enclosed in quills *3 inch in length, painted 
so as to be impermeable to light; of these, 7 did not be- 
come at all inclined towards the light, but 3 of them were 
slightly bent more or less transversely with respect to the line 
of light, and these might perhaps have been altogether ex- 
cluded; one alone was slightly bowed towards the light. 
Painted quills, "25 inch in length, were placed over the summits 
of 4 other cotyledons ; of these, one alone remained upright, a 
second was slightly bowed, and the two others as much bowed 
to the light as the free seedlings in the same pots. These two 
latter cases, considering that the caps were ' 25 in length, are 
inexplicable. 

Lastly, the summits of 8 cotyledons were coated with flexible 
and highly transparent gold-beaters' skin, and all became as 
much bowed to the light as the free seedlings. The summits of 
9 other cotyledons were similarly coated with gold-beaters' skin, 
which was then painted to a depth of between '25 and - 3 inch, 
so as to be impermeable to light; of these 5 remained upright, 
and •! were well bowed to the light, almost or quite as well as 






Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 47ft 

the free seedlings. These latter four cases, as well as the two 
in the last paragraph, offer a strong exception to the rule that 
the illumination of the upper part determines the curvature of 
the lower part. Nevertheless, 5 of these 8 cotyledons remained 
quite upright, although their lower halves were fully illuminated 
all the time; and it would almost be a prodigy to find five free 
seedlings standing vertically after an exposure for several hours 
to a lateral light. 

The cotyledons of Avena, like those of Phalaris, when growing 
in soft, damp, fine sand, leave an open crescentric furrow on the 
shaded side, after bending to a lateral light ; and they become 
bowed beneath the surface at a depth to which, as we know, 
light cannot penetrate. The arcs of the chords of the buried 
bowed portions formed in two cases angles of 20° and 21° with 
the perpendicular. The open furrows on the shaded side were, 
in four cases, *008, '016, "024, and '024 of an inch in breadth. 

Brassica oleracea (Common Eed). — It will Ijere be shown that 
the upper half of the hypocotyl of the cabbage, when illuminated 
by a lateral light, determines the curvature of the lower half. 
It is necessary to experimeutise on young seedlings about half 
an inch or rather less in height, for when grown to an inch and 
upwards the basal part ceases to bend. We first tried painting 
'ihe hypocotyls with Indian ink, or cutting off their summits for 
various lengths ; but these experiments are not worth giving, 
though they confirm, as far as they can be trusted, the results 
of the following ones. These were made by folding gold-beaters' 
skin once round the upper halves of young hypocotyls, and 
painting it thickly with Indian ink or with black grease. As 
a control experiment, the same transparent skin, left unpainteel, 
was folded round the upper halves of 12 hypocotyls ; and these 
all became greatly curved to the light, excepting one r which was 
only moderately curved. Twenty other young hypocotyls had 
the skin round their upper halves painted, whilst their lower 
halves were left quite uncovered. These seedlings were then 
exposed, generally for between 7 and 8 h., in a box blackened 
within and open in front, either before a south-west window or 
a paraffin lamp. This exposure was amply sufficient, as was 
shown by the strongly-marked heliotropism of all the free seed- 
lings in the same pots; nevertheless, some were left exposed 
to the light for a much longer time. Of the 20 hwnocotyls 
thus treated, 14 remained quite upright, and 6 became slightly 
bowed to the light ; but 2 of these latter cases were not really 



180 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

exceptions, for on removing the skin the paint was found im- 
perfect and was penetrated by many small transparent spaces 
on the side which faced the light.. Moreover,, in two other cases 
the painted skin did not extend quite halfway down the hypo- 
cotyl. Altogether there was a wonderful contrast in the several 
pots between these 20 hypocotyls and the other many free 
seedlings, which were all greatly bowed down to their bases m 
the direction of the light, some being almost prostrate on the 
ground. 

The most successful trial on any one day (included in the 
above results) is worth describing in detail. Six young seed- 
lings were selected, the hypocotyls of which were nearly "45 inch, 
excepting one, which was " 6 inch in height, measured from the 
bases of their petioles to the ground. Their upper halves, 
judged as accurately as could be done by the eye, were folded 
once round with gold-beaters' skin, and this was painted 
thickly with Indian ink. They were exposed in an otherwise 
darkened room before a bright paraffin lamp, which stood on 
a level with the two pots containing the seedlings. They 
were first looked at after an interval of 5 h. 10 m., and five 
of the protected hypocotyls were found quite , erect, the sixth 
beiug very slightly inclined to the light ; whereas all the many 
L.*ee seedlings in the same two pots were greatly bowed 
to the light. They were again examined after a continuous 
exposure to the light of 20 h. 35 m. ; and now the contrast 
between the two sets was wonderfully great ; for the free seed- 
lings had their hypocotyls extended almost horizontally in the 
direction of the light, and were curved down to the ground; 
whilst those with the upper halves protected by the painted 
skin, but with their lower halves fully exposed to the light, still 
remained quite upright, with the exception of the one which 
retained the same slight inclination to the light which it had 
before. This latter seedling was found to have been rather 
badly painted, for on the side facing the light the red colour 
of the hypocotyl could be distinguished through the paint. 

We next tried nine older seedlings, the hypocotyls of which 
varied between 1 and 1*6 inch in height. The gold-beaters' 
skin round their upper parts was painted with black grease to 
a depth of only ■ 3 inch, that is, from less than a third to a fourth 
or fifth of their total heights. They were exposed to the light 
for 7 h. 15 m. ; and the result showed that the whole of the 
sensitive zone, which determines the curvature of the lower 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 481 

part, was not protected from the action of the light ; for all 9 
became curved towards it, 4 of them very slightly, 3 moderately, 
and 2 almost as much as the unprotected seedlings. Neverthe- 
less, the whole 9 taken together differed plainly in their degree 
of curvature from the many free seedlings, and from some 
which were wrapped in unpainted skin, growing in the same 
two pots. 

Seeds were covered with about a quarter of an inch of the fine 
sand described under Phalaris ; and when the hypocotyls had 
grown to a height of between '4 and *55 inch, they were exposed 
during 9 h. before a paraffin lamp, their bases being at first 
closely surrounded by the damp sand. They all became bowed 
down to the ground, so that their upper parts lay near to and 
almost parallel to the surface of the soil. On the side of the 
light their bases were in close contact with the sand, which was 
here a very little heaped up; on the opposite or shaded side 
there were open, crescentic cracks or furrows, rather above * 01 
of an inch in width ; but they were not so sharp and regular 
as those made by Phalaris and Avena, and therefore could not 
be so easily measured under the microscope. The hypocotyls 
were found, when the sand was removed on one side, to be 
curved to a depth beneath the surface in three cases of at least 
•1 inch, in a fourth case of '11, and in a fifth of *15 inch. The 
chords of the arcs of the short, buried, bowed portions formed 
angles of between 11° and 15° with the perpendicular. From 
what we have seen of the impermeability of this sand to light, 
the curvature of the hypocotyls certainly extended down to a 
depth where no light could enter; and the curvature must 
have been caused by an influence transmitted from the upper 
illuminated part. 

The lower halves of five young hypocotyls were surrounded by 
unpainted gold-beaters' skin, and these, after an exposure of 8 h. 
before a paraffin lamp, ail became as much bowed to the light 
as the free seedlings. The lower halves of 10 other young 
hypocotyls, similarly surrounded with the skin, were thickly 
painted with Indian ink; their upper and unprotected halves 
became well curved to the light, but their lower and protected 
halves remained vertical in all the cases excepting one, and on 
this the layer of paint was imperfect. This result seems to 
prove that the influence transmitted from the upper part is 
not sufficient to cause the lower part to bend, unless it be at 
the same time illuminated; but there remains the doubt, as in 



482 TRANSMITTED EFFECTS OF LIGHT. Chap. IX 

the case of Phalaris, whether the skin covered with a rathoi 
thick crust of dry Indian ink did not mechanically prevent! 
their curvature. 

Beta vulgaris. — A few analogous experiments were tried on 
this plant, which is not very well adapted for the purpose, as the 
basal part of the hypocotyl, after it has grown to above half an 
inch in height, does not bend much on exposure to a lateral 
light. Four hypocotyls were surrounded close beneath their 
petioles with strips of thin tin-foil, ■ 2 inch in breadth, and they 
remained upright all day before a paraffin lamp ; two others 
were surrounded with strips '15 inch in breadth, and one of 
these remained upright, the other becoming bowed ; the band- 
ages in two other cases were only "1 inch in breadth, and both 
of these hypocotyls became bowed, though one only slightly, 
towards the light. The free seedlings in the same pots were 
all fairly well curved towards the light; and during the follow- 
ing night became nearly upright. The pots were now turned 
round and placed before a window, so that the opposite sides 
of the seedlings were exposed to the light, towards which all 
the unprotected hypocotyls became bent in the course of 7 h. 
Seven out of the 8 seedlings with bandages of tin-foil remained 
upright, but one which had a bandage only - 1 inch in breadth, 
became curved to the light. On another occasion, the upper 
halves of 7 hypocotyls were surrounded with painted gold- 
beaters' skin ; of these 4 remained upright, and 3 became a little 
curved to the light: at the same time 4 other seedlings sur- 
rounded with unpainted skin, as well as the free ones in the 
same pots, all became bowed towards the lamp, before which 
they had been exposed during 22 hours. 

lladicles of Sinapis alba. — The radicles of some plants are 
indifferent, as far as curvature is concerned, to the action of 
light ; whilst others bend towards and others from it.* Whether 
these movements are of any service to the plant is very doubtful, 
at least in the case of subterranean roots ; they probably result 
from the radicles being sensitive to contact, moisture, and gravi- 
tation, and as a consequence to other irritants which are never 
naturally encountered. The radicles of Sinapis alba, when 
immersed in water and exposed to a lateral light, bend from it, 
or are apheliotropic. They become bent for a length of aboui 
4 mm. from their tips. To ascertain whether this movement 



* Sachs, 'Physiologie Yegetale,' 1868, p. 44. 



itaAr. IS. TRANSMITTED EFFECTS OF LIGHT. 483 

generally occurred, 41 radicles, which had germinated in damp 
sawdust, were immersed in water and exposed to a lateral light; 
and they all, with two doubtful exceptions, became curved from 
the light. At the same time the tips of 54 other radicles, 
similarly exposed, were just touched with nitrate of silver. 
They were blackened for a length of from - 05 to "07 mm., and 
probably killed ; but it should be observed that this did not 
check materially, if at all, the growth of the upper part; for 
several, which were measured, increased in the course of only 
8-9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles 
one case was doubtful, 25 curved themselves from the light in 
the normal manner, and 28, or more than half, were not in the 
least apheliotropic. There was a considerable difference, which 
we cannot account for, in the results of the experiments tried 
towards the end of April and in the middle of September. 
Fifteen radicles (part of the above 54) were cauterised at the 
former period and were exposed to sunshine, of which 12 failed 
to be apheliotropic, 2 were still apheliotropic, and 1 was doubt- 
ful. In September, 39 cauterised radicles were exposed to a 
northern light, being kept at a proper temperature ; and now 
23 continued to be apheliotropic in the normal manner, and 
only 16 failed to bend from the light. Looking at the aggregate 
results at both periods, there can be no doubt that the de- 
struction of the tip for less than a millimeter in length destroyed 
in more than half the cases their power of moving from the 
light. It is probable that if the tips had been cauterised for 
the length of a whole millimeter, all signs of apheliotropism 
would have disappeared. It may be suggested that although 
the application of caustic does not stop growth, yet enough may 
be absorbed to destroy the power of movement in the upper 
part; but this suggestion must be rejected, for we have seen 
and shall again see, that cauterising one side of the tip of various 
kinds of radicles actually excites movement. The conclusion 
.seems inevitable that sensitiveness to light resides in the tip 
of the radicle of Sinapis alba ; and that the tip when thus 
stimulated transmits some influence to the upper part, causing 
it to bend. The case in this respect is parallel with that of 
the radicles of several plants, the tips of which are sensitive to 
contact and to other irritants, and, as will be shown in tha 
eleventh chapter, to gravitation. 



i£4 CONCLUDING EEMAEKS AND Chap. IX. 

Concluding Kemarks and Summary of Chapter. 

We do not know whether it is a general rule with 
seedling plants that the illumination of the upper 
part determines the curvature of the lower part. But 
as this occurred in the four species examined by us, 
belonging to such distinct families as the Gramineee, 
Cruciferse, and Chenopodeae, it is probably of common 
occurrence. It can hardly fail to be of service to seed- 
lings, by aiding them to find the shortest path from 
the buried seed to the light, on nearly the same 
principle that the eyes of most of the lower crawling 
animals are seated at the anterior ends of their bodies. 
It is extremely doubtful whether with fully developed 
plants the illumination of one part ever affects the 
curvature of another part. The summits of 5 young 
plants of Asparagus officinalis (varying in height be- 
tween l'l and 2*7 inches, and consisting of several 
short internodes) were covered with caps of tin-foil 
from 03 to 035 inch in depth ; and the lower un- 
covered parts became as much curved towards a lateral 
light, as were the free seedlings in the same pots. 
Other seedlings of the same plant had their summits 
painted with Indian ink with the same negative result. 
Pieces of blackened paper were gummed to the edges 
and over the blades of some leaves on young plants of 
Trojoseolum majus and Ranunculus ficaria ; these were 
then placed in a box before a window, and the petioles 
of the protected leaves became curved towards the 
light, as much as those of the unprotected leaves. 

The foregoing cases with respect to seedling plants 
have been fully described, not only because the trans- 
mission of any effect from light is a new physiological 
fact, bufbecause we think it tends to modify somewhat 
the current views on heliotropic movements. Until 



Chap. IX. „ SUMMARY OF CHAPTER. 48S 

lately such movements were believed to result simply 
from increased growth on the shaded side. At preseiil 
it is commonly admitted * that diminished light in- 
creases the turgescence of the cells, or the extensibility 
of the cell-walls, or of both together, on the shaded 
side, and that this is followed by increased growth. 
But Pfeffer has shown that a difference in the tur- 
gescence on the two sides of a pulvinus, — that is, an 
aggregate of small cells which have ceased to grow at 
an early age, — is excited by a difference in the amount 
of light received by the two sides; and that move- 
ment is thus caused without being followed by in- 
creased growth on the more turgescent side.t All 
observers apparently believe that light acts directly 
on the part which bends, but we have seen with the 
above described seedlings that this is not the case. 
Their lower halves were brightly illuminated for hours, 
and yet did not bend in the least towards the light, 
though this is the part which under ordinary circum- 
stances bends the most. It is a still more striking 
fact, that the faint illumination of a narrow stripe on 
one side of the upper part of the cotyledons of Phalaris 
determined the direction of the curvature of the lower 
part ; so that this latter part did not bend towards the 
bright light by which it had been fully illuminated, 



* Emil Godlewski has given 63, 123, &c. Frank has also 

('Rot. Z.itung,' 1879, Nos. 6-9) insisted ('Die ^aturliche w'a- 

au excellent account (p. 120) of gerechte Richtung von Pflau- 

tlie present state of the question. zentheilen,' 1870, p. 53) on the 

See also Vines in ' Arbeiten des important part which the pulvini 

Rot. Inst, in Wurzburg,' 1878, R. of the leaflets of compound Laves 

ii. pp. 114-147. Hugo de Vries play in placing tie leaflets in a 

has recently published a still proper po,-ition wi'h respect to the 

more important article on this light. This holds ^ood, especially 

subject : ' Rot. Zeitung,' Dec. 19th with the leave^of climbing plants, 

and 26th, 1879. which are carried, into all sorts 

t ' Die Periodischen Rewegun- of positions, ill -adapted for the 

gen der Blattorgane,' 1875, pp. 7, action of the light. 



£86 CONCLUDING EEMAEKS AND Chap. IX. 

but obliquely towards one side where only a little 
light entered. These results seem to imply the pre- 
sence of some matter in the upper part which is acted 
on by light, and which transmits its effects to the 
lower part. It has been shown that this transmission 
is independent of the bending of the upper sensitive 
part. We have an analogous case of transmission in 
Drosera, for when a gland is irritated, the basal and 
not the upper or intermediate part of the tentacle 
bends. The flexible and sensitive filament of Dionaea 
likewise transmits a stimulus, without itself bending ; 
as does the stem of Mimosa. 

Light exerts a powerful influence on most vege- 
table tissues, and there can be no doubt that it 
generally tends to check their growth. But when the 
two sides of a plant are illuminated in a slightly 
different degree, it does not necessarily follow that 
tne bending towards the illuminated side is caused by 
changes in the tissues of the same nature as those 
which lead to increased growth in darkness. We 
know at least that a part may bend from the light, 
and yet its growth may not be favoured by light. 
This is the case with the radicles of Sinapis alba, which 
are plainly apheliotropic ; nevertheless, they grow 
quicker in darkness than in light.* So it is with 
many aerial roots, according to Wiesner ;| but there 
are other opposed cases. It appears, therefore, that 
light does not determine the growth of apheliotropic 
parts in any uniform manner. 

We should bear in mind that the power of bending 
to the light is highly beneficial to most plants. There 



* Francis Darwin, ' Uber das Heft iii., 1880, p. 521. 
Wachvtlium negativ heliotiopi- f ' Sitzb. der k. AUad. der "Wia 

seller Wurzeln': ' Arbeiten des sensch' (Vitnua), 1880, p. 12. 
But. Inst, in Wurzbui",' B. ii . 



Ohaf. IX. SUMMARY OF CHAPTER. 487 

is therefore no improbability in this power having been 
specially acquired. In several respects light seems to 
act on plants in nearly the same manner as it does 
on animals by means of the nervous system.* With 
seedlings the effect, as we have just seen, is trans- 
mitted from one part to another. An animal may be 
excited to move by a very small amount of light ; and 
it has been shown that a difference in the illumination 
of the two sides of the cotyledons of Phalaris, which 
could not be distinguished by the human eye, sufficed 
to cause them to bend. It has also been shown that 
there is no close parallelism between the amount of 
light which acts on a plant and its degree of curva- 
ture; it was indeed hardly possible to perceive any 
difference in the curvature of some seedlings of Phalaris 
exposed to a light, which, though dim, was very much 
brighter than that to which others had been exposed. 
The retina, after being stimulated by a bright light, 
feels the effect for some time ; and Phalaris continued 
to bend for nearly half an hour towards the side which 
had been illuminated. The retina cannot perceive 
a dim light after it has been exposed to a bright one ; 
and plants which had been kept in the daylight 
during the previous day and morning, did not move 
so soon towards an obscure lateral light as did others 
which had been kept in complete darkness. 

Even if light does act in such a manner on the 
growing parts of plants as always to excite in them 
a tendency to bend towards the more illuminated 
side — a supposition contradicted by the foregoing- 
experiments on seedlings and by all apheliotropic 



* Sachs has made some striking See his paper ' I7?ber ortho-trope 

remarks to the same effect with und plagiotrope Pflanzentheile, 

respect to the various stimuli 4 Arb. cles. Bot. Inst, in Wiirzburg- 

o-hich excite movement in plants. 1879 B. ii. p. 282. 
32 



188 CONCLUDING KEMAKKS AND Chap. IX. 

organs — yet the tendency differs greatly m different 
species, and is variable in degree in the individuals of 
the same species, as may be seen in almost any pot 
of seedlings of a long cultivated plant.* There is 
therefore a basis for the modification of this tendency 
to almost any beneficial extent. That it has been 
modified, we see in many cases : thus, it is of more 
importance for insectivorous plants to place their 
leaves in the best position for catching insects than 
to turn their leaves to the light, and they have 
no such power. If the stems of twining plants were 
to bend towards the light, they would often be drawn 
away from their supports ; and as we have seen they 
do not thus bend. As the stems of most other plants 
are heliotropic, we may feel almost sure that twining 
plants, which are distributed throughout the whole 
vascular series, have lost a power that their non- 
climbing progenitors possessed. Moreover, with Ipo- 
moea, and probably all other twiners, the stem of the 
young plant, before it begins to twine, is highly helio- 
tropic, evidently in order to expose the cotyledons or 
the first true leaves fully to the light. With the Ivy the 
stems of seedlings are moderate] y heliotropic, whilst 
those of the same plants when grown a little older 



* Str;isburger has shown in his the light. Some individuals, more- 
interesting work (' Wirkung des over, appear to be indifferent to 
Lichtes . . . an f Sell warm spore n,' the light; and those of different 
1878), that the movement of the species behave very differently, 
swarm-spores of various lowly The brighter the light, the 
organised plants to a lateral light straighter is their course. They 
is influenced by their stage of exhibit also for a s-hort time the 
development, by the temperature after-effects of light. In all these 
to which they are subjected, by r< spt cts they reemble the higher 
the degree of illumination under plants. See, also, Stahl, * Ueber 
which they have been raised, and den einfluss der Lichts auf die 
by other unknown causes ; so that Bewegungs - erscheinungeu der 
the swarm-spores of the same Schwarmsporen ' Verb. d. phys.- 
speoies may move across the field med. Geselsshalft in Wiirzburg, 
of the microscope either to or from B. xii. 1878. 






Cu A v. IX. SUMMARY OF CHAPTER. 489 

are apheliotropic. Some tendrils which consist of 
modified leaves — organs in all ordinary cases strongly 
diaheliotropic — have been rendered apheliotropic, and 
their tips crawl into any dark crevice. 

Even in the case of ordinary heliotropic movements,. 
it is hardly credible that they result directly from 
the action of the light, without any special adaptation. 
We may illustrate what we mean by the hygroscopic 
movements of plants : if the tissues on one side of an 
organ permit of rapid evaporation, they will dry 
quickly and contract, causing the part to bend to this 
side. Now the wonderfully complex movements of 
the pollinia of Orchis pyramidalis, by which they clasp 
the proboscis of a moth and afterwards change their 
position for the sake of depositing the pollen-masses 
on the double stigma — or again the twisting move- 
ments, by which certain seeds bury themselves in 
the ground* — follow from the manner of drying of 
the parts in question ; yet no one will suppose that 
these results have been gained without special adapta- 
tion. Similarly, we are led to believe in adaptation 
when we see the hypocotyl of a seedling, which contains 
chlorophyll, bending to the light ; for although it thus 
receives less light, being now shaded by its own coty- 
ledons, it places them — the more important organs — in 
the best position to be fully illuminated. The hypo- 
cotyl may therefore be said to sacrifice itself for the 
good of the cotyledons, or rather of the whole plant. 
But if it be prevented from bending, as must some- 
times occur with seedlings springing up in an en- 
tangled mass of vegetation, the cotyledons themselves 
bend so as to face the light ; the one farthest off rising 



* Francis "Darwin, * On the Fly- actions Linn. See.,' series ii. vol. i. 
groscopic Mechanism,' &c, 'Trans- p. 149, 1876. 



£90 CONCLUDING REMAKES AND Chap. IX 

up, and that nearest to the light sinking down, 01 
both twisting laterally.* We may, also, suspect iiiat 
the extreme sensitiveness to light of the upper part 
of the sheath-like cotyledons of the Graminese, and 
their power of transmitting its effects to the lower 
part, are specialised arrangements for finding the 
shortest path to the light. With plants growing on 
a bank, or thrown prostrate by the wind, the manner 
in which the leaves move, even rotating on their own 
axes, so that their upper surfaces may be again directed 
to the light, is a striking phenomenon. Such facts 
are rendered more striking when we remember that 
too intense a light injures the chlorophyll, and that 
the leaflets of several Leguminosse when thus exposed 
bend upwards and present their edges to the sun, thus 
escaping injury. On the other hand, the leaflets of 
Averrhoa and Oxalis, when similarly exposed, bend 
downwards. 

It was shown in the last chapter that heliotropism 
is a modified form of circumnutation ; and as every 
growing part of every plant circumnutates more or less, 
we can understand how it is that the power of bending 
to the light has been acquired by such a multitude 
of plants throughout the vegetable kingdom. The 
manner in which a circumnutating movement — that 
is, one consisting of a succession of irregular ellipses 
or loops — is gradually converted into a rectilinear 
course towards the light, has been already explained. 
First, we have a succession of ellipses with their 
longer axes directed towards the light, each of which 



* Wiesrier has made remarks to tracted from B. lxxvii. (187s) 

Dearly the same effect with respect Sitb. der k. Akud. der Wisseusch 

to leaves : * Die imdulirende Nu- Wien. 
tation der Intornodieu,' p. G, ex- 



Chap. IX. SUMMARY OF CHAPTER. 49.1 

is described nearer and nearer to its source ; then the 
loops are drawn ont into a strongly pronounce d zigzag 
line, with here and there a small loop still formed. 
At the same time that the movement towards the light 
is increased in extent and accelerated, that in the 
opposite direction is lessened and retarded, and. at last 
stopped. The zigzag movement to either side is 
likewise gradually lessened, so that finally the course 
becomes rectilinear. Thus under the stimulus of a 
fairly bright light there is no useless expenditure of 
force. 

As with plants every character is more or less 
variable, there seems to be no great difficulty in be- 
lieving that their circumnutating movements may 
have been increased or modified in any beneficial 
manner by the preservation of varying individuals. 
The inheritance of habitual movements is a necessary 
contingent for this process of selection, or the survival 
of the fittest ; and we have seen good reason to believe 
that habitual movements are inherited by plants. In 
the case of twining species the circumnutating move- 
ments have been increased in amplitude and rendered 
more circular ; the stimulus being here an internal 
or innate one. With sleeping plants the movements 
have been increased in amplitude and often changed 
in direction ; and here the stimulus is the alternation 
of light and darkness, aided, however, by inheritance. 
In the case of heliotropism, the stimulus is the unequal 
illumination of the two sides of the plant, and this 
determines, as in the foregoing cases, the modifica- 
tion of the circumnutating movement in such a manner 
that the organ bends to the light. A plant which 
has been rendered heliotropic by the above means, 
might readily lose this tendency, judging from the 
cases already given, as soon as it became useless 01 



±92 CONCLUDING KEMAEKS. Chap. IX. 

injurious. A species which has ceased to be helio- 
tropic might also be rendered apheliotropic by the 
preservation of the individuals which tended to cir- 
cumnutate (though the cause of this and most other 
variations is unknown) in a direction more or less 
opposed to that whence the light proceeded. In like 
manner a plant might be rendered diaheliotropie. 



Chap X MOVEMENTS EXCITED BY GKAVITATION. 493 



CHAPTER X. 

Modified Circumnutation : Movements excited by Gkavitatio:?. 

Means of observation - -Apogeotropism — Cytisus — Verbena — Beta — 
Gradual conversion of the movement of circumnutation into apogeo- 
tropis.ni in Kubus, Lilium, Phalaris, Avena, and Bra^sica — Apogeo- 
tropisni retarded by heliotropism — Effected by the aid of joints 
or pulviui — Movements of flower-peduncles of Oxalis — GJeneral 
remarks on apogeotropism — Geotropism — Movements of rarlicles — 
Burying of seed-capsules — Use of process — Trifolium subterraneum 
— Arachis — Amphicarpsea — Dia geotropism — Conclusion. 

Our object in the present chapter is to show that 
geotropism, apogeotropism, and diageotropism are mo- 
dified forms of circumnutation. Extremely fine fila- 
ments of glass, bearing two minute triangles of paper, 
were fixed to the summits of young stems, frequently 
to the hypocotyls of seedlings, to flower-peduncles, 
radicles, &c, and the movements of the parts were 
then traced in the manner already described on 
vertical and horizontal glass-plates. It should be 
remembered that as the stems' or other parts become 
more and more oblique with respect to the glasses, the 
figures traced on them necessarily become more and 
more magnified. The plants were protected from light, 
excepting whilst each observation was being made, and 
then the light, which was always a dim one, was 
allowed to enter so as to interfere as little as possible 
with the movement in progress ; and we did not detect 
any evidence of such interference. 

When observing the gradations between circumnu 



494 MODIFIED CIKC UMNUTATION. Chap X. 

tation and heliotropisrn, we had the great advantage of 
being able to lessen the light ; bnt with geotropisni 
analogous experiments were of course impossible. 
We could, however, observe the movements of stemH 
placed at first only a little from the perpendicular, in 
which case geotropism did not act with nearly so much 
power, as when the stems were horizontal and at right 
angles to the force. Plants, also, were selected which 
were but feebly geotropic or apogeotropic, or had 
become so from having grown rather old. Another 
plan was to place the stems at first so that they pointed 
30 or 40 degrees beneath the horizon, and then apo- 
geotropism had a great amount of work to do before 
the stem was rendered upright ; and in this case 
ordinary circumnutation was often not wholly oblite- 
rated. Another plan was to observe in the evening 
plants which during the day had become greatly 
curved heliotropically ; for their stems under the gra- 
dually waning light very slowly became upright through 
the action of apogeotropism ; and in this case modified 
circumnutation was sometimes well displayed. 

Apogeotropism. — Plants were selected for observation almost 
by chance, excepting that they were taken from widely different 
families. If the stem of a plant which is even moderately 
sensitive to apogeotropism be placed horizontally, the upper 
growing part bends quickly upwards, so as to become perpen- 
dicular; and the line traced by joining the dots successively 
made on a glass -plate, is generally almost straight. For in- 
stance, a young Cytims fragrans, 12 inches in height, was placed 
so that the stem projected 10° beneath the horizon, and its 
course was traced during 72 h. At first it bent a very little 
downwards (Fig. 182), owing no doubt to the weight of the 
stem, as this occurred with most of the other plants observed, 
though, as they were of course circumnutating, the short down- 
ward lines were often oblique. After three-quarters of an hour 
the stem began to curve upwards, quickly during the first two 
hours, but much more slowly during the afternoon and night, 



ClIAP. X. 



APOGEOTKOPISM. 



495 



Fig. 182. J 

T 
% 



{W' 



and on the following day. During the second night it foil 
a little, and circumnutated 
during the following day; but it 
also moved a short distance to 
the right, which was caused by 
a little light haying been ac- 
cidentally admitted on this side. 
The stem was now inclined 
60 3 above the horizon, and had 
therefore risen 70°. "With time 
allowed it would probably have 
become upright, and no doubt 
would have continued circum- 
nutating. The sole remarkable 
feature in the figure here given 
is the straightness of the course 
pursued. The stem, however, 
did not move upwards at an 
equable rate, and it sometimes 
stood almost or quite still. 
Such periods probably represent 
attempts to circumnutate in a 
direction opposite to apogeo- 
tropism. 

The herbaceous stem of a 
Verbena melindres (?) laid hori- 
zontally, rose in 7 h. so much 
that it could no longer be 
observed on the vertical glass 
which stood in front of the plant. 
The long line which was traced 
was almost absolutely straight. 
After the 7 h. it still continued 

to rise, but now circumnutated Cy ^/^ an : s L apog , e °n™ pic ' 
slightly. On the following day 



it stood upright, and circum- 
nutated regularly, as shown in 
Tig. 82, given in the fourth 
chapter. The stems of several 
other plants which were highly 
sensitive to apogeotropism rose 
up in almost straight lines, and 



move 
ment of stem from 10° beneath t' 
60° above horizon, traced on vei 
tical glass, from 8.30 a.m. Marcf 
12th to 10.30 p.m. 13th. The sub- 
sequent circumnutating movement 
is likewise shown up to 6.45 A.M. 
on the 15th. Nocturnal course 
represented, as usual, by a broken 
liDe. Movement not greatly mag- 
nified, and tracing reduced to two- 
thirds of original scale. 



496 



MODIFIED CTRCUMNUTATION. 



Chap. X 



then suddenly began to circumnutate. A partially etiolated 

and somewhat old hypocotyl 



Fig. 183. M2:s8'p.w.. 



12:3 



Btp.m. 



7.°a.m.29 l 



8°55'am.28 r ? £ 

Seta wilwris: apogcotropic movement 
ofhypocotyl from 19° beneath horizon 
to a vertical position, with subsequent 
circunmutation, traced on a vertical 
and on a horizontal glass-plate, from 
8.28 A.M. Sept. 28th to 8.40 A.M. 29th. 
Figure reduced toone-thirdof original 
seal ;. 



of a seedling cabbage (21 
inches in height) was so 
sensitive that when placed 
at an angle of only 23° from 
the perpendicular, it became 
vertical in 33 minutes. As 
it could not have been 
strongly acted upon by 
apogeotropism in the above 
slightly inclined position, 
we expected that it would 
have circumnutated, or at 
least have moved in a zig- 
zag course. Accordingly, 
dots were made every 3 
minutes; but, when these 
were joined, the line was 
nearly straight. After this 
hypocotyl had become up- 
right it still moved onwards 
for half an hour in the same 
general direction, but in a 
zigzag manner. During the 
succeeding 9 h. it circum- 
nutated regularly, and de- 
scribed 3 large ellipses. In 
this case apogeotropism, 
although acting at a very 
unfavourable angle, quite 
overcame the ordinary cir- 
cumnutating movement. 

The hypocotyls of Beta 
vulgaris are highly sensitive 
to apogeotropism. One was 
placed so as to project 19° 
beneath the horizon ; it fell 
at first a very little (see 
Fig. 183), no doubt owing 
to its weight : but as it was 
circumnutating the line was 



Chap. X. APOGEOTROPISM. 41)7 

oblique During the next 3 h. 8 m. it rose in a nearly straight 
line, passing through an angle of 109°, and then (at 12.3 p.m.) 
stood upright. It; continued for 55 m. to move in the same 
general direction beyond, the perpendicular, but in a zigzag- 
course. It returned also in a zigzag line, and then circumnu- 
tated regularly, describing three large ellipses during the 
remainder of the day. It should be observed that the ellipses 
in this figure are exaggerated in size, relatively to the length of 
the upward straight line, owing to the position of the vertical 
and horizontal glass-plates. Another and somewhat old hypo- 
cotyl was placed so as to stand at only 31° from the perpen- 
dicular, in which position apogeotropism acted on it with little 
force, and its course accordingly was slightly zigzag. 

The sheath-like cotyledons of Phalaris Canadensis are ex- 
tremely sensitive to apogeotropism. One was placed so as to 
project 40° beneath the horizon. Although it was rather old 
and 1*3 inch in height, it became vertical in 4 h. 30 m., having 
passed through an angle of 130° in a nearly straight line. It then 
suddenly began to circumnutate in the ordinary manner. The 
cotyledons of this plant, after the first leaf has begun to pro- 
trude, are but slightly apogeotropic, though they still continue 
to circumnutate. One at this stage of development was placed 
horizontally, and did not become upright even after 13 h., and its 
course was slightly zigzag. So, again, a rather old hypocotyl 
of Cassia tora (li inch in height) required 28 h. to become up- 
right, and its course was distinctly zigzag ; whilst younger hypo- 
cotyls moved much more quickly and in a nearly straight line. 

When a horizontally placed stem or other organ rises in a 
zigzag line, we may infer from the many cases given in our 
previous chapters, that we have a modified form of circumnu- 
tation ; but when the course is straight, there is no evidence 
of circumnutation, and any one might maintain that this latter 
movement had been replaced by one of a wholly distinct kind. 
This view seems the more probable when (as sometimes 
occurred with the hypocotyls of Brassica and Beta, the stems of 
Cucurbita, and the cotyledons of Phalaris) the part in question 
after bending up in a straight course, suddenly begins to circum- 
nutate to the full extent and in the usual manner. A fairly 
good instance of a sudden change of this kind — that is, from a 
nearly straight upward movement to one of circumnutation — 
is shown in Fig. 183 ; but more striking instances were occa- 
sionally observed with Beta, Brassica, and Phalaris. 

We will now describe a few cases in which it may be 



£98 MODIFIED CTRCUMNUTATION. Chap. X 

seen how gradually circumnutation becomes changed into apogeo- 
tropism, nnder circumstances to be specified 

lg ' in each instance. 

Eubus idceus (hybrid). — A young plant, 11 

g 'inches in height, growing in a pot, was placed 

g horizontally; and the upward movement was 

i3, traced during nearly 70 h. ; but the plant, 
though growing vigorously, was not highly 

re -5 sensitive to apogeotropism, or it was not 

«* * capable of quick movement, for during the 

£> a above time it rose only 67°. We may see in 

2 r° the diagram (Fig. 184) that during the first 
w> ° day of 12 h. it rose in a nearly straight line, 
•g 5 When placed horizontally, it was evidently 
« ° circumnutating, for it rose at first a little, 
J S notwithstanding the weight of the stem, and 
~ g then sank down ; so that it did not start on 
•J o its permanently upward course until 1 h. 
£ Z 25 m. had elapsed. On the second day, by 
rt g which time it had risen considerably, and 
° g when apogeotropism acted on it with somewhat 
o 2 less power, its course during 152 h. was clearly 

3 5/5 zigzag, and the rate of the upward movement 
S "^ was not equable. During the third day, also 
w -£ of 15! h., when apogeotropism acted on it 
° « with still less power, the stem plainly circum- 
§ * nutated, for it moved during this day 3 times 
£ ao up and 3 times down, 4 times to the left and 
| S 4 to the right. But the course was so complex 
g 5 that it could hardly be traced on the glass. 
I"" -1 We can, however, see that the successively 
§ | formed irregular ellipses rose higher and 
§Jg higher. Apogeotropism continued to act on 
* ^ the fourth morning, as the stem was still 
ri 1 ^ rising, though it now stood only 23° from the 
"f^o perpendicular. In this diagram the several 
w* -1 stages may be followed by which an almost 

I | rectilinear, upward, apogeotropic course first 

' becomes zigzag, and then changes into a 

[ J circumnutating movement, with most of the 

» c=i successively formed, irregular ellipses directed 

upwards. 
Lilium aitratum. — A plant 23 inches in height was placed 



Chap. X 



APOGEOTROPISM. 



499 



Fig. 185. 




horizontally, and the upper part of the stem rose 58° in 16 h.. 
in the manner shown in the accom- 
panying diagram (Fig. 185). We here 
see that during the whole of the 
second day of 15* h., the stem plainly 
circumnutated whilst bending upwards 
through apogeotropism. It had still 
to rise considerably, for when the last 
dot in the figure was made, it stood 
32° from an upright position. 

PJialaris Canariemis. — A cotyledon 
of this plant (1"3 inch in height) has 
already been described as rising in 
4 h. 30 m. from 40° beneath the hori- 
zon into a vertical position, passing 
through an angle of 130° in a nearly 
straight line, and then abruptly be- 
ginning to circumnutate. Another 
somewhat old cotyledon of the same 
height (but from which a true leaf 
had not yet protruded), was similarly 
placed at 40° beneath the horizon. For 
the first 4 h. it rose in a nearly straight 
course (Fig. 186), so that by 1.10 p.m. 
it was highly inclined, and now apo- 
geotropism acted on it with much less 
power than before, and it began to 
zigzag. At 4.15 p.m. (i.e. in 7 h. from 
the commencement) it stood vertically, 
and afterwards continued to circum- 
nutate in the usual manner about the 
same spot. Here then we have a 
graduated change from a straight up- 
ward apogeotropic course into circum- Lilium auratum : apoge-> 
nutation, instead of an abrupt change, tropic movement of stem, 
as in the former case. 

Avena sativa.—Tlie sheath-like coty- 
ledons, whilst young, are strongly apo- 
geotropic ; and some which were placed 
at 45° beneath the horizon rose 90° in 
7 or 8 h. in lines almost absolutely 
straight. An oldish cotyledon, from which the first leaf began to 




traced on a vertical glass 
during 2 days and 2 
nights, from 10.40 A.M. 
March 18th to 8 A.M. 
20th. Figure reduced to 
one-half of the original 
scale. 



500 



MODIFIED CIRCUMXUTATIOX. 



Chat. X 



Fig. 186. 




Phalans Canariensis ; ipogeotropic move- 
ment of cotyledon, traced on a vertical 
and horizontal glass, from 9. 10 A.M. Sept. 
19th to 9 a M. 20th. Figure here re- 
duced to one-fifth of original ?cale. 



protrude whilst the fol- 
lowing observations were 
being made, was placed 
at 10° beneath the horizon, 
and it rose only 59° in 
24 h. It behaved rather 
differently from any other 
plant, observed by us, for 
during the first 4^ h. it 
rose in a line not far from 
straight ; during the next 
6a h. it circumnutatecl, 
that is, it descended and 
again ascended in a 
strongly marked zigzag 
course; it then resumed 
its upward movement in 
a moderately straight line, 
and, with time allowed, 
no doubt would have be- 
come upright. In this 
case, after the first 4 J h., 
ordinary circumnutation 
almost completely con- 
quered for a time apogeo- 
tropisrn. 

Brass tea oleracea. — The 
hypocotyls of several 
young seedlings placed 
horizontally, rose up ver- 
tically in the course of 6 
or 7 h. in nearly straight 
lines. A seedling which 
had grown in darkness to 
a height of 2i inches, and 
was therefore rather old 
and not highly sensitive, 
was placed so that the 
hypocotyl projected at be- 
tween 30° and 40° beneath 
the horizon. The upper 
part alone became curved 



Chap. X. 



APOGEOTROPISM. 



501 



upwards, and rose during the first 3h. 10 m. in a nearly straight 
line (Fig. 187); but it was not 
possible to trace the upward move- 
ment on the vertical glass for the 
first 1 h. 10 m., so that the nearly 
straight line in the diagram ought 
to have been much longer. During 
the next 11 h. the hypocotyl circuni- 
nutated, describing irregular figures, 
each of which rose a little above 
the one previously formed. During 
the night and following early morn- 
ing it continued to rise in a zigzag 
course, so that apogeotropism was 
still acting. At the close of our ob- 
servations, after 23 h. (represented 
by the highest dot in the diagram) 
the hypocotyl was still 32° from 
the perpendicular. There can be 
little doubt that it would ulti- 
mately have become upright by 
describing an additional number 
of irregular ellipses, one above the 
other. 

Apogeotropism retarded by TJelio- 
tropism. — When the stem of any 
plant bends during the day towards 
a lateral light, the movement is 
opposed by apogeotropism ; but as 
the light gradually wanes in the 
evening the latter power slowly 
gains the upper hand, and draws Brass 
the stem back into a vertical 
position. Here then we have a 
good opportunity for observing how 
apogeotropism acts when very 
nearly balanced by an opposing 
force. For instance, the plumule 
of Tropceolum majus (see former 
J?ig. 175) moved towards the dim 
evening light in a slightly zigzag 
b'ne until 6.45 p.m., it then returned on its course until 




ica oleracea: apogeotropic 
movement of hypocotyl, traced 
on vertical glass, from 9.20 
a.m. Sept. 12th to 8.30 a.m. 
loth. The upper part of the 
figure is more magnified than 
the lower part. If the whole 
course had been traced, the 
straight upright line would 
have been much longer. Figure 
here reduced to one-third of 
the original scale. 



502 MODIFIED CIKCUMNUTATION. Chap. X 

10.40 p.m., during which time it zigzagged and described an 
sllipse of considerable size. The hypocotyl of Brassica oleracea 
(see former Fig. 173) moved in a straight line to the light until 
5.15 p.m., and then from the light, making in its backward 
course a great rectangular bend, and then returned for a short 
distance towards the former source of the light ; no observa- 
tions were made after 7.10 p.m., but during the night it re- 
covered its vertical position. A hypocotyl of Cassia tora moved 
in the evening in a somewhat zigzag line towards the failing- 
light until 6 p.m., and was now bowed 20° from the perpendi- 
cular ; it then returned on its course, making before 10.30 p.m. 
four great, nearly rectangular bends and almost completing an 
ellipse. Several other analogous cases were casually observed, 
and. in all of them the apogeotropic movement could be seen to 
consist of modified circumnutation. 

Apogeotropic Movements effected by the aid of joints or pidvini. 
— Movements of this kind are well known to occur in the 
Graniinese, and are effected by means of the thickened bases 
of their sheathing leaves; the stem within being in this part 
thinner than elsewhere.* According to the analogy of all other 
pulvini, such joints ought to continue circumnutating for a 
long period, after the adjoining parts have ceased to grow. We 
therefore wished to ascertain whether this was the case with 
the Graminese ; for if so, the upward curvature of their stems, 
when extended horizontally or laid prostrate, would be explained 
in accordance with our view — namely, that apogeotropism 
results from modified circumnutation. After these joints have 
curved upwards, they are fixed in their new position by increased 
growth along their lower sides. 

Lolium prrenTie. — A young stem, 7 inches in height, consist- 
ing of 3 internodes, with the flower-head not yet protruded, 
was selected for observation. A long and very thin glass fila- 
ment was cemented horizontally to the stem close above the 
second joint, 3 inches above the ground. This joint was subse- 
quently proved to be in an active condition, as its lower side 
swelled much through the action of apogeotropism (in the 
manner described by De Vries) after the haulm had been 
fastened down for 2-1 h. in a horizontal position. The pot was 



* This structure lias been re- die Aufrichtung des gelagerten 
cently dts<ribed by De Vries in Getreides,' in ' Landwirthschaft- 
an interesting article, 'Ueber liclie. Jahrliiiclier,' 1880, p. 473. 



CaiP. X. APOGEOTROPISM. 503 

bo placed that the end of the filament stood beneath the 2-inch 
object glass of a microscope with an eye-piece micrometer, each 
division of which equalled ^ of an inch. The end of the fila- 
ment was repeatedly observed during 6 h., and was seen to be 
in constant movement ; and it crossed 5 divisions of the micro- 
meter (yig inch) in 2 h. Occasionally it moved forwards bv 
jerks, some of which were yoVo mcn m length, and then slowly 
retreated a little, afterwards again jerking forwards. These 
oscillations were exactly like those described under Brassica 
and Dionsea, but they occurred only occasionally. We may 
therefore conclude that this moderately old joint was continually 
circumnutating on a small scale. 

Alopecurus pratensis.—A young plant, 11 inches in height, with 
the flower-head protruded, but with the florets not yet expanded, 
had a glass filament fixed close above the second joint, at a 
height of only 2 inches above the ground. The basal internode, 
2 inches in length, was cemented to a stick to prevent any 
possibility of its circumnutatiug. The extremity of the filament, 
which projected about 50° above the horizon, was often observed 
during 24 h. in the same manner as in the last case. Whenever 
looked at, it was always in movement, and it crossed 30 divisions 
of the micrometer (^ inch) in 3| h. ; but it sometimes moved 
at a quicker rate, for at one time it crossed 5 divisions in Is h. 
The pot had to be moved occasionally, as the end of the filament 
travelled beyond the field of vision ; but as far as we could 
judge it followed during the daytime a semicircular course ; 
and it certainly travelled in two different directions at right 
angles to one another. It sometimes oscillated in the same 
manner as in the last species, some of the jerks forwards being 
as much as y^ of an inch. We may therefore conclude that 
the joints in this and the last species of grass long continue to 
circumnutate ; so that this movement would be ready to be 
converted into an apogeotropic movement, whenever the stem 
was placed in an inclined or horizontal position. 

Movements of the Flower-peduncles of Oxalis carnosa, due to 
apogeotropism and other fortes. — The movements of the main 
peduncle, and of the three or four sub-peduncles which each 
main peduncle of this plant bears, are extremely complex, and 
are determined by several distinct causes. Whilst the flowers 
are expanded, both kinds of peduncles circumnutate about the 
same spot, as we have seen (Fig 91) in the fourth chapter. 
But soon after the flowers have begun to wither the sub- 



501 MODIFIED CIECUMNUTATION. Chap. X 

peduncles bend downwards, and this is due to epinasty; foi 
on two occasions when pots were laid horizontally, the sub- 
peduncles assumed the same position relatively to the main 
peduncle, as would have been the case if they had remained 
upright; that is, each of them formed with it an angle of 
about 40°. If they had been acted on by geotropism or aphelio- 
tropism (for the plant was illuminated from above), they would 
have directed themselves to the centre of the earth. A main 
peduncle was secured to a stick in an upright position, and one 
of the upright sub-peduncles which had been observed circum- 
nutating whilst the flower was expanded, continued to do so for 
at least 24 h. after it had withered. It then began to bend 
downwards, and after 36 h. pointed a little beneath the horizon. 
A new figure was now begun (A, Fig. 188), and the sub-peduncle 
was traced descending in a zigzag line from 7.20 p.m. on the 19th 
to 9 a.m. on the 22nd. It now pointed almost perpendicularly 
downwards, and the glass filament had to be removed and 
fastened transversely across the base of the young capsule. 
"We expected that the sub-peduncle would have been motionless 
in its new position ; but it continued slowly to swing, like a 
pendulum, from side to side, that is, in a plane at right angles 
to that in which it had descended. This circumnutating move- 
ment was observed from 9 a.m. on 22nd to 9 a.m. 24th, as shown 
at B in the diagram. We were not able to observe this par- 
ticular sub-peduncle any longer; but it would certainly have 
gone on circumnutating until the capsule was nearly ripe (which 
requires only a short time), and it would then have moved 
upwards. 

The upward movement (C, Fig. 188) is effected in part by the 
whole sub-peduncle rising in the same manner as it had pre- 
viously descended through epinasty — namely, at the joint where 
united to the main peduncle. As this upward movement 
occurred with plants kept in the dark and in whatever position 
the main peduncle was fastened, it could not have been caused 
by heliotropism or apogeotropism, but by hyponasty. Besides 
this movement at the joint, there is another o: a very different 
kind, for the sub-peduncle becomes upwardly bent in the middle 
part. If the sub-peduncle happens at the time to be inclined 
much downwards, the upward curvature is so great that the 
whole forms a hook. The upper end bearing the capsule, thus 
always places itself upright, and as this recurs in darkness, and 
in whatever position the main peduncle may have been secured, 



Chap. X. 



APOGEOTROPISM. 



505 



fche upwai'd curvature cannot be due to heliotropism or hypo- 
nasty, but to apogeotropism. 



; 



Fig. 188. 




% 




Oxalh carnosa : movements of flower-peduncle, traced on a vertical g/iass 
A, apinastic downward movement ; B, circumnutation whilst depend* 
int; rertically ; C, subsequent upward movement, due to apogeotropism 
and hvponastv combined 



506 MODIFIED CIRCUMNUTATION. Chap. X. 

In order to trace this upward movement, a filament was fixed 
to a sub-peduncle bearing a capsule nearly ripe, which was 
beginning to bend upwards by the two means just described. Its 
coarse was traced (see C, Fig. 188) during 53 h., by which time 
it had become nearly upright. The course is seen to be strongly 
zigzag, together with some little loops. We may therefore con- 
clude that the movement consists of modified circumnutation. 

The several species of Oxalis probably profit in the following 
manner by their sub-peduncles first bending downwards and 
then upwards. They are known to scatter their seeds by the 
burs'ing of the capsule; the walls of which are so extremely 
thin, like silver paper, that they would easily be permeated by 
rain. But as soon as the petals wither, the sepals rise up and 
enclose the young capsule, forming a perfect roof over it as 
soon as the sub-peduncle has bent itself downwards. By its 
subsequent upward movement, the capsule stands when ripe 
at a greater height above the ground by twice the length of the 
sub-peduncle, than it did when dependent, and is thus able 
to scatter its seeds to a greater distance. The sepals, which 
enclose the ovarium whilst it is young, present an additional 
adaptation by expanding widely when the seeds are ripe, so as 
not to interfere with their dispersal. In the case of Oaalis 
aretosella, the capsules are said sometimes to bury themselves 
under loose leaves or moss on the ground, but this cannot occur 
with those of 0. carnosa, as the woody stem is too high. 

Oxalis acetosella. — The peduncles are furnished with a joint in 
Fig.-189. . 




Oxalis acetosella : course pursued by the upper part of a peduncle, whilst 
rising, traced from 11 a.m. June 1st to 9 a.m. 3rd. Figure here re- 
duced to one-half of the original scale. 

the middle, so that the lower part answers to the main peduncle, 






Chap. X. APOGEOTROPISM. 507 

and the upper part to one of the sub-peduncles of 0. cai nosa. 
The upper part bends downwards, after the flower has begun 
to wither, and the whole peduncle then forms a hook; that 
this bending is due to epinasty we may infer from the case of 
0. carnosa. When the pod is nearly ripe, the upper part 
straightens itself and becomes erect ; and this is due to hypo- 
nasty or apogeotropism, or both combined, and not to helio- 
tropism, for it occurred in darkness. The short, hooked part of 
the peduncle of a cleistogamic flower, bearing a pod nearly ripe, 
was observed in the dark during three days. The apex of the 
pod at first pointed perpendicularly down, but in the course of 
three days rose 90°, so that it now projected horizontally. The 
course during the two latter days is shown in Fig. 189; and 
it may be seen how greatly the peduncle, whilst rising, circum- 
nutated. The lines of chief movement were at right angles 
to the plane of the originally hooked part. The tracing was 
not continued any longer ; but after two additional days, the 
peduncle with its capsule had become straight and stood 
upright. 

Concluding Remarks on Apogeotropism. — When apo- 
geotropism is rendered by any means feeble, it acts, 
as shown in the several foregoing cases, by increasing 
the always present circumnutating movement in a 
direction opposed to gravity, and by diminishing that 
in the direction of gravity, as well as that to either 
side. The upward movement thus becomes unequal 
in rate, and is sometimes interrupted by stationary 
periods. Whenever irregular ellipses or loops are still 
formed, their longer axes are almost always directed 
in the line of gravity, in an analogous manner as 
occurred with heliotropic movements in reference to 
the light. As apogeotropism acts more and more 
energetically, ellipses or loops cease to be formed, and 
the course becomes at first strongly, and then less and 
less zigzag, and finally rectilinear. From thip grada- 
tion in the nature of the movement, and more especially 
from all growing parts, which alone (except when pul- 
vini are present) are acted on by apogeotropism, con- 



508 MODIFIED CIRCUMNUTATION. Chap. X, 

fcinually circLinnutating, we may conclude that even 
a rectilinear course is merely an extremely modified 
form of circumnutation. It is remarkable that a stem 
or other organ which is highly sensitive to apogeo- 
tropism, and which has bowed itself rapidly upwards 
in a straight line, is often carried beyond the vertical, 
as if by momentum. It then bends a little backwards 
to a point round which it finally circumnutates. Two 
instances of this were observed with the hypocotyls of 
Beta vulgaris, one of which is shown in Fig. 183, and 
two other instances with the hypocotyls of Brassica. 
This momentum-like movement probably results from 
the accumulated effects of apogeotropism. For the 
sake of observing how long such after-effects lasted, 
a pot with seedlings of Beta was laid on its side in the 
dark, and the hypocotyls in 3 h. 15 m. became highly 
inclined. The pot, still in the dark, was then placed 
upright, and the movements of the two hypocotyls were 
traced ; one continued to bend in its former direction, 
now in opposition to apogeotropism, for about 37 m., 
perhaps for 48 m. ; but after 61 m. it moved in an 
opposite direction. The other hypocotyl continued 
to move in its former course, after being placed 
upright, for at least 37 m. 

Different species and different parts of the same 
species are acted on by apogeotropism in very dif- 
ferent degrees. Young seedlings, most of which cir- 
cumnutate quickly and largely, bend upwards and 
become vertical in much less time than do any older 
plants observed by us; but whether this is due to 
their greater sensitiveness to apogeotropism, or merely 
to their greater flexibility we do not know. A hypo- 
cotyl of Beta traversed an angle of 109° in 3 h. 8 m., 
and a cotyledon of Phalaris an angle of 130° in 4 h. 
30 m. On the other hand, the stem of a herbaceous 



Chap. X. A.POGEOTROPISM. 509 

Verbena rose 90° in about 24 h. ; that of Eubus 67°, 
in 70 h. ; that of Oytisus 70°, in 72 h. ; that of a young 
American Oak only 37°, in 72 h. The stem of a 
young Gyperus altemifolius rose only 11° in 96 h. ; 
the bending being confined to near its base. Though 
the sheath-like cotyledons of Phalaris are so extremely 
sensitive to apogeotropism, the first true leaves which 
protrude from them exhibited only a trace of this 
action. Two fronds of a fern, Nephrodium molle, both 
of them young and one with the tip still inwardly 
curled, were kept in a horizontal position for 46 h., 
and during this time they rose so little that it was 
doubtful whether there was any true apogeotropic 
movement. 

The most curious case known to us of a difference 
in sensitiveness to gravitation, and consequently of 
movement, in different parts of the same organ, is that 
offered by the petioles of the cotyledons of Ipomoea 
leptophylla. The basal part for a short length where 
united to the undeveloped hypocotyl and radicle is 
strongly geotropic, whilst the whole upper part is 
strongly apogeotropic. But a portion near the blades 
of the cotyledons is after a time acted on by epinasty 
and curves downwards, for the sake of emerging in the 
form of an arch from the ground; it subsequently 
straightens itself, and is then again acted on by apo- 
geotropism. 

A branch of Gucurbita ovifera, placed horizontally, 
moved upwards during 7 h. in a straight line, until it 
stood at 40° above the horizon ; it then began to cir- 
cumnutate, as if owing to its trailing nature it had no 
tendency to rise any higher. Another upright branch 
was secured to a stick, close to the base of a tendril, 
and the pot was then laid horizontally in the c'ark. 
In this position the tendril eircumnutated and mad# 



510 MODIFIED CIRCUMNUTATION. Chap. X. 

several large ellipses during 14 h., as it likewise did 
on the following day; but during this whole time it 
was not in the least affected by apogeotropism. On the 
other hand, when branches of another Cucurbitaceous 
plant, Echinocytis lobata, were fixed in the dark so that 
the tendrils depended beneath the horizon, these began 
immediately to bend upwards, and whilst thus moving 
they ceased to circumnutate in any plain manner; 
but as soon as they had become horizontal they re- 
commenced to revolve conspicuously.* The tendrils 
of Passifiora gracilis are likewise apogeotropic. Two 
branches were tied down so that their tendrils pointed 
many degrees beneath the horizon. One was observed 
for 8 h., during which time it rose, describing two 
circles, one above the other. The other tendril rose 
in a moderately straight line during the first 4 h., 
making however one small loop in its course ; it then 
stood at about 45° above the horizon, where it circum- 
nutated during the remaining 8 h. of observation. 

A part or organ which whilst young is extremely 
sensitive to apogeotropism ceases to be so as it grows 
old ; and it is remarkablej as showing the independence 
of this sensitiveness and of the circumnutating move- 
ment, that the latter sometimes continues for a time 
after all power of bending from the centre of the earth 
has been lost. Thus a seedling Orange bearing only 
3 young leaves, with a rather stiff stem, did not curve 
in the least upwards during 24 h. whilst extended 
horizontally ; yet it circumnutated all the time over 
a small space. The hypocotyl of a young seedling 
of Cassia tor a, similarly placed, became vertical in 
12 h. ; that of an older seedling, 1£ inch in height, 



* For details see • The Movements and Habits of Climbing Plants, 
1875, p. 131. 



Chap. X. APOGEOTEOPISM. 511 

became so in 28 h. ; and that of another still older 
one, 1J inch in height, remained horizontal d lring 
two days, but distinctly circumnutated during this 
whole time. 

When the cotyledons of Phalaris or Avena are laid 
horizontally, the uppermost part first bends upwards, 
and then the lower part ; consequently, after the lower 
part has become much curved upwards, the upper part 
is compelled to curve backwards in an opposite direc- 
tion, in order to straighten itself and to stand ver- 
tically ; and this subsequent straightening process is 
likewise due to apogeotropism. The upper part of 
8 young cotyledons of Phalaris were made rigid by 
being cemented to thin glass rods, so that this part 
could not bend in the least ; nevertheless, the basal 
part was not prevented from curving upward. A stem 
or other organ which bends upwards through apogeo- 
tropism exerts considerable force; its own weight, 
which has of course to be lifted, was sufficient in 
almost every instance to cause the part at first to bend 
a little downwards ; but the downward course was 
often rendered oblique by the simultaneous circum- 
nutating movement. The cotyledons of Avena placed 
horizontally, besides lifting their own weight, were 
able to furrow the soft sand above them, so as to leave 
little crescentic open spaces on the lower sides of their 
bases; and this is a remarkable proof of the force 
exerted. 

As the tips of the cotyledons of Phalaris and Avena 
bend upwards through the action of apogeotropism 
before the basal part, and as these same tips when 
excited by a lateral light transmit some influence to 
the lower part, causing it to bend, we thought that 
the same rule might hold good with apogeotropism. 
Consequently, the tips of 7 cotyledons of Phalaris w ere 



512 MODIFIED CIECUMNUTATION. Chap X 

cut off for a length in three cases of *2 inch and in 
the four other cases of *14, -12, *1, and *07 inch. But 
these cotyledons, after being extended horizontally, 
bowed themselves upwards as effectually as the un- 
niutilated specimens in the same pots, showing that 
sensitiveness to gravitation is not confined to their tips. 

Geotkopism. 

This movement is directly the reverse of apogeo- 
tropism. Many organs bend downwards through epi- 
nasty or apheliotropism or from their own weight ; but 
we have met with very few cases of a downward move- 
ment in sub-aerial organs due to geotropism. We 
shall, however, give one good instance in the following 
section, in the case of Trifolium siibterraneum, and 
probably in that of Aracliis liypogsea. 

On the other hand, all roots which penetrate the 
ground (including the modified root-like petioles of 
Megarrhiza and Ipomoea leptophylla) are guided in their 
downward course by geotropism ; and so are many 
aerial roots, whilst others, as those of the Ivy, appear 
to be indifferent to its action. In our first chapter the 
movements of the radicles of several seedlings were 
described. We may there see (Fig. 1) how a radicle 
of the cabbage, when pointing vertically upwards so 
as to be very little acted on by geotropism, circum- 
nutated ; and how another (Fig. 2) which was at first 
placed in an inclined position bowed itself downwards 
in a zigzag line, sometimes remaining stationary for a 
time. Two other radicles of the cabbage travelled 
downwards in almost rectilinear courses. A radicle of 
the bean placed upright (Fig. 20) made a great sweep 
and zigzagged ; but as it sank downwards and was 
more strongly acted on by geotropism, it moved in an 



Chap. X. GEOTEOPISM. 513 

almost straight course. A radicle of Cucurbita, directed 
upwards (Fig. 26), also zigzagged at first, and de- 
scribed small loops ; it then moved in a straight line. 
Nearly the same result was observed with the radicles 
of Zea mays. But the best evidence of the intimate 
connection between circumnutation and geotropism 
was afforded by the radicles of Phaseolus, Vicia, and 
Quercus, and in a less degree by those of Zea and 
^Esculus (see Figs. 18, 19, 21, 41, and 52) ; for when 
these were compelled to grow and slide down highly 
inclined surfaces of smoked glass, they left distinctly 
serpentine tracks. 

Tlie Burying of Seed-capsules: Trifolium subterraneum. — The 
flower-heads of this plant are remarkable from producing only 
3 or 4: perfect flowers, which are situated exteriorly. All the 
other many flowers abort, and are modified into rigid points, 
with a bundle of vessels running up their centres. After a time 
5 long, elastic, claw-like projections, which represent the divi- 
sions of the calyx, are developed on their summits. As soon as 
the perfect flowers wither they bend downwards, supposing the 
peduncle to stand upright, and they then closely surround its 
upper part. This movement is due to epinasty, as is likewise 
the case with the flowers of T. repens. The imperfect central 
flowers ultimately follow, one after the other, the same course. 
Whilst the perfect flowers are thus bending down, the whole 
peduncle curves downwards and increases much in length, 
until the flower-head reaches the ground. Vaucher * says that 
when the plant is so placed that the heads cannot soon reach 
the ground, the peduncles grow to the extraordinary length of 
from 6 to 9 inches. In whatever position the branches may be 
placed, the upper part of the peduncle at first bends vertically 
upwards through heliotropism ; but as soon as the flowers 
begin to wither the downward curvature of the whole peduncle 
commences. As this latter movement occurred in complete 
darkness, and with peduncles arising from upright and from 
dependent branches, it cannot be due to apheliotropism or to 
epinasty, but must be attributed to geotropism. Nineteen 



* 'Hist Phys. des Plantes d'Europe,' torn. ii. 1811, p. 106. 



514 MODIFIED CIKCUMNUTATION Chap. X. 

upright flower-heads, arising from branches in all sorts of posi- 
tions, on plants growing in a warm greenhouse, were marked 
with thread, and after 24 h. six of them were vertically depen- 
dent; these therefore had travelled through 180° in this time. 
Ten were extended sub-horizontally, and these had moved 
through about 90°. Three very young peduncles had as yet 
moved only a little downwards, but after an additional 24 h. 
were greatly inclined. 

At the time when the flower-heads reach the ground, the 
younger imperfect flowers in the centre are still pressed closely 
together, and form a conical projection ; whereas the perfect and 
imperfect flowers on the outside are upturned and closely sur- 
round the peduncle. They are thus adapted to offer as little 
resistance, as the case admits of, in penetrating the ground, 
though the diameter of the flower-head is still considerable. 
The means by which this penetration is effected will presently 
be described. The flower-heads are able to bury themselves in 
common garden mould, and easily in sand or in fine sifted 
cinders packed rather closely. The depth to which they pene- 
trated, measured from the surface to the base of the head, was 
between i and § inch, but in one case rather above 0'6 inch. 
With a plant kept in the house, a head partly buried itself in 
sand in 6 h. : after 3 days only the tips of the reflexed calyces 
were visible, and after 6 days the whole had disappeared. But 
with plants growing out of doors we believe, from casual obser- 
vations, that they bury themselves in a much shorter time. 

After the heads have buried themselves, the central aborted 
flowers increase considerably in length and rigidity, and 
become bleached. They gradually curve, one after the other, 
upwards or towards the peduncle, in the same manner as 
did the perfect flowers at first. In thus moving, the long claws 
on their summits carry with them some earth. Hence a flower- 
head which has been buried for a sufficient time, forms a rather 
large ball, consisting of the aborted flowers, separated from one 
another by earth, and surrounding the little pods (the product 
of the perfect flowers) which lie close round the upper part of 
the peduncle. The calyces of the perfect and imperfect flowers 
are clothed with simple and multicellular hairs, which have the 
power of absorption ; for when placed in a weak solution of 
carbonate of ammonia (2 gr. to 1 oz. of water) their proto- 
plasmic contents immediately became aggregated and afterwards 
displayed the usual slow movements. This clover generally 



Chap. X. GEOTROPISM. 515 

grows in dry soil, but whether the power of absorption by the 
hairs on the buried flower-heads is of any importance to them 
we do not know. Only a few of the flower-heads, which from 
their position are not able to reach the ground and bury them- 
selves, yield seeds ; whereas the buried ones never failed, as far 
as we observed, to produce as many seeds as there had been 
perfect flowers. 
We will now consider the movements of the peduncle whilst 



Fig. 190. 



JI°(tm.22 n . d 




7 c am.22f? 



TrifoHum subterraneum : downward movement of peduncle from 19°beneath 
the horizon to a nearly vertically dependent position, traced from 
1 I A.M. July 22nd to the morning of 25th. Glass filament fixed 
transversely across peduncle, at base of flower-head. 

curving down to the ground. We have seen in Chap. IV., 
Fig. 92, p. 225, that an upright young flower-head circumnu- 
tated conspicuously; and that this movement continued after 
the peduncle had begun to bend downwards. The same 
peduncle was observed when inclined at an angle of 19° abc ?e 
the horizon, and it circumnutated during two da} s. Anothei 



516 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



which was already curved 36° beneath the horizon, was observed 
from 11 a.m. July 22nd to the 27th, by which latter date it 
had become vertically dependent. Its course during the first 
1.2 h. is shown in Fig. 190, and its position on the three 
succeeding mornings until the 25th. 
when it was nearly vertical. During 
the first day the peduncle clearly 
circumnutated, for it moved 4 times 
down and 3 times up; and on each 
succeeding day, as it sank downwards, 
the same movement continued, but 
was only occasionally observed and 
was less strongly marked. It should 
m: cir- be stat ^ d that these peduncles were 
cumoutating movement of observed under a double skylight in 




peduncle, whilst the flower- 
head was burying itself in 



the house, and that they generally 
moved downwards very much more 



traced from 8 A.M. July 
26th to 9 A.M. on 27th. 
Glass filament fixed trans- 
vei-sely across peduncle, 
'near flower- head. 



Fig. 192. 



sand, with the reflexed tips 

of the calvx still visible ; slowly than those on plants growing 
out of doors or in the greenhouse. 

The movement of another vertically 
dependent peduncle with the flower- 
head standing half an inch above the 
ground, was traced, and again when 
it first touched the ground; in both cases irregular ellipses 
were described every 4 or 5 h. A peduncle on a plant which 
had been brought into the house, 
moved from an upright into a ver- 
tically dependent position in a 
single clay; and here the course 
during the first 12 h. was nearly 
straight, but with a few well-rnarkt- d 
zigzags which betrayed the essential 
nature of the" movement. Lastly,, 
the circumnntation of a peduncle 
was traced during 51 h. whilst in 
the act of burying itself obliquely 
in a little heap of sand. After it had buried itself to such a 
depth that the tips of the sepals were alone visible, the above 
figure (Fig. 191) was traced during 25 h. When the flower- 
head had completely disappeared beneath the. sand, another 
tracing was made during 11 h. 45 m. (Fig. 192) ; and here again 
we see that the peduncle was circumnutating. 




Trijo'ium suhterraneum : move- 
ment of same peduncle, with 
flower-head completely buried 
beneath the sand ; traced from 
8 A.M. to 7. 1 5 P.M. on July 29th. 



Chap. X. GEOTPOPISM. 517 

Any one who will observe a flower-head burying itself, will be 
convinced that the rocking novement, due to the continued 
circumnutation of he peduncle, plays an important part in the 
act. Considering that the flower-heads are very light, that the 
peduncles are long, thin, and flexible, and that they arise from 
flexible branches, it is incredible that an objc ct as blunt as one 
of these flower-heads could penetrate the ground by means of 
the growing force of the peduncle, unless it were aided by the 
locking movement. After a flower-liead has penetrated the 
ground to a small depth, another and efficient ageicy comes into 
play; the central rigid aborted flowers, each terminating in five 
long claws, curve up towards the peduncle; and in doing so 
can hardly fail to drag the head down to a greater depth, aided 
as this action is by the circumnutating movement, which con- 
tinues after the flower-head has completely buried itself. The 
aborted flowers thus act something like the hands of the mole, 
which force the. earth backwards and the body forwards. 

It is well known that the seed-capsules of various widely 
distinct plants either bury themselves in the ground, or are 
produced from imperfect flowers developed beneath the surface. 
Besides the present case, two other well-marked instances will 
be immediately, given. It is probable that one chief good thus 
gained is the protection of the seeds from animals which prey on 
them. In'the case of T. suiter raneum, the seeds are not only 
concealed by being buried, but are likewise protected by being 
closely surrounded by the rigid, aborted flowers. We may the 
mure confidently infer that protection is here aimed at, because 
the seeds of several species in this same genus are protected in 
other ways ;* namely, by the swelling and closure of the calyx, 
or by the persistence and bending down of the standard-petal, &c. 
But the most curious instance is that of T. globosum, in which 
the upper flowers are sterile, as in T. subterraneum, but are here 
developed into large brushes of hairs which envelop and protect 
the seed-bearing flow r ers. Nevertheless, in all these cases the 
capsules, w-ith their seeds, may profit, as Mr. T. Thiselton Dyer 
has remarked,! by their being kept somewhat damr and the 
advantage of such dampness perhaps throws light on the pre- 
sence of the absorbent hairs on the buried flower-heads of T. sub- 
terraneum. According to Mr. Bentham, as quoted by Mr. Dyer, 



* Vancher, 'Hist. Phys. des t See his interesting article in 
Plantes d'Europe,' torn. ii. p. 110. ' Nature,' April 4th. 1878, p. 44(3 



518 MODIFIED CIRCUMNUTATION. Chap. X. 

the prostrate habit of Hdianthemum prostratum " brings the 
capsules in contact with the surface of the ground, postpones 
their maturity, and so favours the seeds attaining a larger size." 
The capsules of Cyclamen and of Oxulis ucetosdla are only occa- 
sionally buried, and this only beneath dead leaves or moss. If 
it be an advantage to a plant that its capsules should be kept 
damp and cool by being laid on the ground, we have in these 
latter cases the first step, from which the power of penetrating 
the ground, with the aid of the. always present movement of 
circumnutation, might afterwards have been gained. 

Arachis hyp< gcea. — The flowers which bury themselves, rise 
from stiff branches a few inches above the ground, and stand 
upright. After they have fallen off, the gynophore, that is the 
part which supports the ovarium, grows to a great length, even 
to 3 or 4 inches, and bends perpendicularly downwards. It 
resembles closely a peduncle, but has a smooth and pointed 
apex, which contains the ovules, and is at first not in the least 
enlarged. The apex after reaching the ground penetrates it, in 
one case observed by us to a depth of 1 inch, and in another 
to 0*7 inch. It there becomes developed into a large pod. 
Flowers which are seated too high on the plant for the gyno- 
phore to reach the ground are said * never to produce pods. 

The movement of a young gynophore, rather under an inch 
in length and vertically dependent, was traced during 46 h. by 
means of a glass filament (with sights) fixed transversely a 
iittle above the apex. It plainly drcumnutated (Fig. 193) 
whilst increasing in length and growing downwards. It was 
then raised up, so as to be extended almost horizontally, and 
the terminal part curved itself downwards, fo'lowing a nearly 
straight course during 12 h., but with one attempt to circum- 
nutate, as shown in Fig. 194. After 24 h. it had become nearly 
vertical. Whether the exciting cause of the downward move- 
ment is geotropism or apheliotropism was not ascertained ; but 
probably it is not apheliotropism, as all the gynophores grew 
straight down towards the ground, whilst the light in the hot- 
house entered from one side as well as from above. Another 
and older gynophore, the apex of which had nearly reached the 
ground, was observed during 3 days in the same manner as the 
first-mentioned short one; and it was found to be always circum- 
nutatiug. During the first 34 h. it described a figure which 



* ' Gard. Chronicle,' 1857, p. 566. 



Chap. X. 



GEOTROPISM. 



519 



represented four ellipses. Lastly, a long gynophore, the apex of 
which had buried itself to the depth of about half an inch, was 

Fig. 194 



Pig. 193 




Arachis h>/pogce<x : circum- 
nutation of vertically- 
dependent young gyno- 
phore, traced on a ver- 
tical glass from 10 A.M. 
July 31st to 8 A.M. Aug. 
2nd. 



Arachis hypojcea : down- 
ward movement of same 
young gynophore, after 
being extended horizon- 
tally; traced on a vertical 
glass from 8.30 A.M. to 
8.30 p.m. Aug. 2nd. 



pulled up and extended horizontally: it quickly began to curve 
downwards in a zigzag line ; but on the following day the ter- 
- ' 34. 



520 MODIFIED CIRCUMNUTATION. Chap. X. 

minal bleached portion was a little shrivelled. As the gyno- 
phores are rigid and arise from stiff branches, and as they 
terminate in sharp smooth points, it is probable that they could 
penetrate the ground by the mere force of growth. But this 
action must be aided by the circumnutating movement, for fine 
sand, kept moist, was pressed close round the apex of a gyno- 
phore which had reached the ground, and after a few hours it 
was surrounded by a narrow open crack. After three weeks 
this gynophore was uncovered, and the apex was found at a 
depth of rather above half an inch developed into a small, white, 
oval pod. 

Amphicarpcea monoica. — This plant produces long thin shoots, 
which twine round a support and of course circumnutate. 
Early in the summer shorter shoots are produced from the 
lower parts of the plant, which grow perpendicularly downwards 
and penetrate the ground. One of these, termiLating in a 
minute bud, was observed to bury itself in sand to a depth of 
0*2 inch in 21 h. It was lifted up and fixed in an inclined 
position about 25° beneath the horizon, being feebly illuminated 
from above. In this position it described two vertical ellipses 
in 24 h. ; but on the following day, when brought into the house; 
it circumnutattd only a very little round the same spot. Other 
branches were seen to penetrate the ground, and were after- 
wards found running like roots beneath the surface for a length 
of nearly two inches, and they had grown thick. One of these, 
after thus running, had emerged into the air. How far circum- 
nutation aids these delicate branches in entering the ground we 
do not know; but the reflexed hairs with which they are clothed 
will assist in the woik. This plant produces pods in the air, 
and others beneath the ground ; which differ greatly in appear- 
ance. Asa Gray says * that it is the imperfect flowers on the 
creeping branches near the base of the plant which produce the 
subterranean pods; these flowers, therefore, must bury them- 
selves like tho.-e of Arachis. But it may be suspected that the 
branches which were seen by us to penetrate the ground also 
produce subterranean flowers and pods. 

DlAGEOTROFTSM. 

Besides geotropism and apogeotropism, there is, 
according t) Frank, an allied form of movement, 



Manual of the Botany of the Northern United States,' 1556. p. 10G. 






Chap. X DIAGEOTKOPISM. 521 

namely, " transverse-geotropism," or diageotropism, as 
we may call it for the sake of matching our other 
terms. Under the influence of gravitation certain 
parts are excited to place themselves more or less 
transversely to the line of its action.* We made no 
observations on this subject, and will here only re- 
mark that the position of the secondary radicles of 
various plants, which extend horizontally or are a 
little inclined downwards, would probably be con- 
sidered by Frank as due to transverse-geotropism. 
As it has been shown in Chap. I. that the secondary 
radicles of Cucurbita made serpentine tracks on a 
smoked glass-plate, they clearly circumnutated, 
and there can hardly be a doubt that this holds 
good with other secondary radicles. It seems there- 
fore highly probable that they place themselves in 
their diageotropic position by means of modified 
circumnutation. 

Finally, we may conclude that the three kinds of 
movement which have now been described and which 
are excited by gravitation, consist of modified circum- 
nutation. Different parts or organs on the same plant, 
and the same part in different species, are thus excited 
to act in a widely different manner. We can see no 
reason why the attraction of gravity should directly 
modify the state of turgescence and subsequent growth 
of one part on the upper side and of another part on 
the lower side. We are therefore led to infer that both 
geotropic, apogeotropic, and diageotropic movements, 
the purpose of which we can generally understand. 



* Elfving has lately described excellent instance of such caove- 
''Arbtiten des Bot. Instituts in ments in the lhizomes of ceitair 
Wuizburg.' B. ii. 1880, p. 489, an plants. 



522 MODIFIED CIKCUMNUTATION. Chap X 

have been acquired for the advantage of the plant by 
the modification of the ever-present movement of 
circnmnutation. This, however, implies that gravi- 
tation produces some effect on the young tissues 
sufficient to serve as a guide to the plant. 



Chap. XL SENSITIVENESS TO GRAVITATION. 523 



CHAPTEE XL 

Localised Sensitiveness to Gravitation, and its Transmitted 
Effects. 

General considerations — \ieia faba, effects of amputating the tips of 
the radicles— regeneration of the tips— Effects of a short exposure 
of the tips to geot'-opic notion and their subsequent amputation — 
Effects of amputating the tips obliquely — Effects of cauterising the 
tips — Effects of grease on the tips — Pisum sativum, tips of radicles 
cauterised transversely, and on their upper and lower sides — 
Phaseolus, cauterisation and grease on the tips — Gossypium — 
Cucurbita, tips cauterised transversely, and on their upper and. 
lower sides — Zea, frps cauterised — Concluding remarks and 
summary of chapter — Advantages of the sensibility to geotropism 
being localised in the tips of the radicles. 

Ciesielski states * that when the roots of Pisnm, 
Lens and Yicia were extended horizontally with their 
tips cut off, they were not acted on by geotropism ; 
but some days afterwards, when a new root-cap and 
vegetative point had been formed, they bent them- 
selves perpendicularly downwards. He further states 
that if the tips are cut off, after the roots have been 
left extended horizontally for some little time, but 
before they have begun to bend downwards, they may 
be placed in any position, and yet will bend as if still 
acted on by geotropism ; and this shows that some 
influence had been already transmitted to the bending 
part from the tip before it was amputated. Sachs 
repeated these experiments ; he cut off a length of 
between -05 and 1 mm. (measured from the apex of the 



* ' Abwartskrumrnung der Wurzel,' Inaug. Dissert. Breslau. 1871, 
p. 29. 



524 SENSITIVENESS TO GEAVITATION. Chap. XL 

vegetative point) of the tips of the radicles of the 
bean (Vicia faba), and placed them horizontally or 
vertically in damp air, earth, and water, with the 
result that they became bowed in all sorts of direc- 
tions.* He therefore disbelieved in Ciesielski's con- 
clusions. But as we have seen with several plants 
that the tip of the radicle is sensitive to contact and 
to other irritants, and that it transmits some influence 
to the upper growing part causing it to bend, there 
seemed to us to be no a priori improbability in 
Ciesielski's statements. We therefore determined to 
repeat his experiments, and to try others on several 
species by different methods. 

Vicia fab i. — Eadicles of this plant were extended horizontally 
either over water or with their lower surfaces just touching it. 
Their tips had previously been cut off, in a direction as accu- 
rately transverse as could be done, to different lengths, measured 
from the apex of the root-cap, and which will be specified in 
each case. Light was always excluded. We had previously 
tried hundreds of unmutilatect radicles under similar circum- 
stances, and found that every one that was healthy becan e 
plainly geotropic in under 12 h. In the case of four radicles 
which had their tips cut off for a length of 1*5 mm., new ruot- 
caps and new vegetative points were re-formed after an interval 
of 3 days 20 h. ; and these when placed horizontally were acted 
on by geotropism. On some other occasions this regeneration 
of the tips and reacquired sensitiveness occurred within a some- 
what shorter time. Therefore, radicles having their tips 
amputated should be observed in from J 2 to 48 h. after the 
operation. 

Four radicles were extended horizon'allj with their lower 
surfaces tonchmg the water, and with their tips cut off for a 
length of only 05 mm. : after 23 h. three of them were still 
horizontal ; after 47 h. one of the three became fairly geotropic; 
and after 70 h. the other two showed a trace of this action. The 
fourth radicle was vertically geotropic after 23 h. ; but by an 



Arbeiten des Bot. Instituts in Wiirzburo;.' Heft. iii. 1878, p. 482. 



(Jhap. XI. TRANSMITTED EFFECTS : VICIA. 525 

accident the root-cap alone and not the vegetative point was 
found to have been amputated ; so that this case formed no real 
exception and might have been excluded. 

Five radicles were extended horizontally like the last, and 
had their tips cut off for a length of I mm. ; after 22-23 h., four 
of them were still horiz ntal, and one was slightly geotropic ; 
after 48 h. the latter had become vertical; a second was also 
somewhat geotropic; two remained approximately horizontal; 
and the last or fifth had grown in a disordered manner, for it 
was inclined upwards at an angle of 6 )° above the horizon. 

Fourteen radicles were extended horizontally at a little height 
over the water with their tips cut off for a length of 1-5 mm. ; 
after 12 h. all were horizontal, whilst five control or standard 
specimens in the same jar were all bent greatly downwards. 
After 24 h. several of the amputated radicles remained hori- 
zontal, but some showed a trace of geotropism, and one was 
plainly geotropic, for it was inclined at 40° beneath the horizon. 

Seven horizontally extended radicles from which the tips had 
been cut off for the unusual length of 2 mm. unfortunate ly were 
not looked at until 35 h. had elapsed ; three were still horizontal, 
but, to our surprise, four were more or less plainly geotropic. 

The radicles in the foregoing cases w r ere measured before their 
tips were amputated, and in the course of 24 h. they had all 
increased greatly in length; but the measurements are not 
worth giving. It is of more importance that Sachs found that 
the rate of growth of the different parts of radicles with 
amputated tips was the same as with unmutilated ones. Alto- 
gether twenty-nine radicles were operated on in the manner 
above described, and of these only a few showed any geotropic 
curvature within 24 h. ; whereas radicles with unmutilated tips 
always became, as already stated, much bent down in less than 
half of this time. The part of the radicle which bends most lies 
at the distance of from 3 to 6 mm. from the tip, and as the 
bending part continues to grow after the operation, there does 
not seem any reason why it should not have been acted on by 
geotropism, unless its curvature depended on some influence 
transmitted from the tip. And we have clear evidence of such 
transmission in Ciesielski's experiments, which we repeated and 
extended in the following manner. 

Beans were embedded in friable peat with the hilum down- 
wards, and after their radicles had grown perpendicularly down 
for a length of from ^ to 1 inch, sixteen were selected which 



526 SENSITIVENESS TO GKAVITATION. Chap. XI 

were perfectly straight, and these were placed horizontally on 
the peat, being covered by a thin layer of it. They were thus 
left for an average period of 1 h. 37 m. The tips were then cut 
off transversely for a length of 1*5 mm., and immediately after- 
wards tiiey were embedded vertically in tbe peat. In 'his position 
geotropism would not tend to induce any curvature, but if some 
influence had already been transmitted from the tip to the part 
which bends most, we might expect that this part would become 
curved in the direction in which geotropism had previously 
acted; for it should be noted that these radicles being now 
destitute of their sensitive tips, would not be prevented by 
geotropism from curving in any direction. The result was that 
of the sixteen vertically embedded radicles, four continued for 
several days to grow straight downwards, whilst twelve became 
more or less bowed laterally. In two of the twelve, a trace of 
curvature was perceptible in 3 h. 30 m., counting from the time 
when they had first been laid horizontally ; and all twelve were 
plainly bowed in 6 h., and still more plainly in 9 h. In every 
one of them the curvature was directed towards the side which 
had been downwards whilst the radicles remained horizontal. 
The curvature extended for a length of from 5 to, in one in- 
stance, 8 mm., measured from the cut-off end. Of the twelve 
bowed radicles five became permanently bent into a right angle ; 
the other seven were at first much less bent, and their curvature 
generally decreased after 24 h., but did not wholly disappear. 
This decrease of curvature would naturally follow, if an ex- 
posure of only 1 h. 37 m. to geotropism, served to modify the 
turgescence of the cells, but not their subsequent growth to 
the full extent. The five radicles which were rectangularly 
bent became fixed in this position, and they continued to grow 
out horizontally in the peat for a length of about 1 inch during 
from 4 to 6 days. By this time new tips had been formed ; and 
it should be remarked that this regeneration occurred slower in 
the peat than in water, owing perhaps to the radicles being 
often looked at and thus disturbed. After the tips had been 
regenerated, geotropism was able to act on them, so that they 
now became bowed vertically downwards. An accurate draw- 
ing (Fig. 195) is given on the opposite page of one of these five 
radicles, reduced to half the natural size. 

We next tried whether a shorter exposure to geotropism 
would suffice to produce an after-effect. Seven radicles were 
extended horizontally for an hour, instead of 1 h. 37 m. as in the 



Chap. XL TRANSMITTED EFFECTS : VICIA. 



527 



former trial; and after their tips (1*5 mm. in length) had been 
amputated, they were placed vertically in damp peat. Of these, 
three were not in the least affected and continued for days to 
grow straight downwards. Four showed after 8 h 30 m. a mere 
trace of curvature in the direction in which they had been acted 
on by geotropism ; and in this respect they differed much from 
those which had been exposed for 
1 h. 37 in., for many of the latter 
were plainly curved in 6 h. The 
curvature of one of these four 
radicles almost disappeared after 
24 h. In the second, the cur- 
vature increased during two days 
and then decreased. The third 
radicle became permanently bent, 
so that its terminal part made an 
angle of about 45° with its original 
vertical direction. The fourth 
radicle became horizontal. These 
two latter radicles continued 
during two more days to grow 
in the peat iu the same directions, 
that is, at an angle of 45° be- Viciafaba: radicle, rectangularly 




neath the horizon and horizon- 
tally. By the fourth morning new 
tips had been re-formed, and now 
geotropism was able to act on 
them again, and they became 
bent perpendicularly downwards, 
exactly as in the case of the 
five radicles described in the 
last paragraph and as is shown in 



bent at A, after the mnpu'ation 
of the tip, due to the previous 
influence of geotropi-m. L, side 
of bean which lay on the peat, 
whilst geotropism acted on the 
radicle. A, point of chief cur- 
vature of the radicle, whilst 
standing vertically downwards. 
B, point of chief curvature after 
the regeneration of the tip, when 
geotropism again acted. C, re- 
generated tip. 



the figure (Fig. 195) here given. 

Lastly, five other radicles were similarly treated, but were ex- 
posed to geotropism during only 45 m. After 8 h. 30 m. only 
one was doubtfully affected; after 24 h. two were just per- 
ceptibly curved towards the side which had bee;i acted on by 
geotropism ; after 48 h. the one first mentioned had a radius of 
curvature of 60 mm. That this curvature was due to the action 
of geotropism during the horizontal position of the radicle, waa 
shown after 4 days, when a new tip had been reformed, fur it 
then grew perpendicularly downwards. We learn from this 



528 SENSITIVENESS TO GKAVITATION. Chai XI 

caee that when the tips are amputated after an exposure to geo- 
tropisui of only 45 m., though a slight influence is sometimes 
transmitted to the adjoining part of the radicle, yet this seldom 
suffices, and then only slowly, to induce even moderately well- 
pronounce 1 curvature. 

In the previously given experiments on 29 horizontally ex- 
tended radicles with their tips amputated, only one grew irre- 
gularly in any marked manner, and this became bowed upwards 
at an angle of 65^. In Ciesielski's experiments the radicles 
could not have grown very irregularly, for if they had done 
so, he could not have spoken confidently of the obliteration 
of all geotropic action. It is therefore remarkable that Sachs, 
who experimented on many radicles with their tips amputated, 
found extremely disordered growth to be the usual result. As 
horizontally extended radicles with amDutated tips are some- 
times acted on slightly by geotropism within a short time, and 
are often acted on plainly after one or two clays, we thought 
that this influence might possibly prevent disordered growth, 
though it was not able to induce immediate curvature. There- 
fore 13 radicle^, of which 6 had their tips amputated trans- 
versely for a length of 1*5 mm., and the other 7 for a length of 
only 0'5 mm., were suspended vertically in damp air, in which 
position they would not be affected by geotropism; but they 
exhibited no great irregularity of growth, whilst observed 
during 4 to 6 days. We next thought that if care were not 
taken in cutting off the tips transversely, one side of the stump 
might be irritated more than the other, either at first or sub- 
sequently during the regeneration of the tip, and that this 
might cause the radicle to bend to one side. It has also been 
shown in Chapter III. that if a thin slice be cut off one side 
of the tip of the radicle, this causes the radicle to bend from 
the sliced side. Accordingly, 30 radicles, with tips amputated 
for a length of 1*5 mm., were allowed to grow perpendicularly 
downwards into water. Twenty of them were amputated at an 
angle of 20 D with a line transverse to their longitudinal axes; 
and such stumps appeared only moderately oblique. The 
remaining ten radicles were amputated at an angle of about 
45°. Under these circumstances no less than 19 out of the 30 
became much distorted in the course of 2 or 3 days. Eleven 
other radicles were similarly treated, excepting that only 1 mm. 
(including in this and all other cases the root-cap) was ampu- 
tated ; and of these only one grew much and two others slightly 






Chap. XI. TRANSMITTED EFFECTS : VICIA. 529 

distorted; so that this amount of oblique amputation was not 
sufficient. Out of the above 30 radicles, only one or two showed 
in the first 24 h. any distortion, but this became plain in the 
19 cases on *.he second day, and still more conspicuous at the 
close of the third day, by which time new tips had been partially 
or completely regenerated. When therefore a new tip is re- 
formed on an oblique stump, it probably is developed sooner on 
one side than on the other: and this in some manner excites 
the adjoining part to bend to one side. Hence it seems probable 
that Sachs unintentionally amputated the radicles on which he 
experimented, not strictly iu a transverse direction. 

This explanation of the occasional irregular growth of radicles 
with amputated tips, U supported by the results of cauterising 
their tips; for often a greater length on one S'de than on the 
other was unavoidably injured or killed. It should be re- 
marked that in the following trials the tips were first dried 
with blotting-paper, and then slightly rubbed with a dry stick 
of nitrate of silver or lunar caustic. A few touches with the 
caustic suffice to kill the root-cap and some of the upper layers 
of cells of the vegetative point. Twenty-seven ladicles, some 
young and very short, others of moderate length, were suspended 
vertically over water, after being thus cauterised. Of these some 
entered the water immediately, and others on the second day. 
The same number of uncauterised radicles of the same age 
were observed as controls. After an interval of three or four 
days the contrast in appearance between the cauterised and 
control specimens was wonderfully great. The controls had 
grown straight downward-, with the exception of the normal 
curvature, which we have called Sachs' curvature. Of the 
27 cauteri eel radicles, 15 had become extremely distorted ; 6 of 
them grew upwards and formed hoops, so that their tips some- 
times came into contact with the bean above ; 5 grew out 
rectangularly to one side ; only a few of the remaining 12 were 
quite straight, and some of these towards the close of our 
observations became hooked at their extreme lower ends. 
Radicles, extended horizontally instead of vertically, with their 
tips cauterised, also sometimes grew distorted, but not so com- 
monly, as far as we could judge, as those suspended vertically; 
for this occurred with only 5 out of 19 radicles, thus treated. 

Instead of cutting off the tips, as in the first set of experi- 
ments, we next tried the effects of touching horizontally ex- 
tended radicles with caustic in the manner just described. But 



530 SENSITIVENESS TO GEAVITATION. Chap XL 

some preliminary remarks must first be made. It may be ob- 
jected that the caustic would injure the radicles and prevent them 
from bending ; but ample evidence was given in Chapter III. 
that touching the tips of vertically suspended radicles with 
caustic on one side, does not stop their ben ling; on the 
contrary, it causes them to bend from the touched side. We 
also tried touching both the upper and the lower sides of the 
tips of some radicles of the bean, extended horizontally in damp 
friable earth. The tips of three were touched with caustic on 
their upper sides, and this would aid their geotropic bending; 
the tips of three were touched on their lower sides, which 
would tend to counteract the bending downwards ; and three 
were left as controls. After 24 h. an independent observer was 
asked to pick out of the nine radicles, the two which were most 
and the two which were least bent ; he selected as the lattei 
two of those which had been touched on their lower sides, and 
as the most bent, two of those which had been touched on the 
upper side. Hereafter analogous and more striking experiments 
with Pisum sativum and Oucurbita ovifera will be given. We 
may therefore safely conclude that the mere application of 
caustic to the tip does not prevent the radicles from bending. 

In the following experiments, tne tips of young horizontally 
extended radicles were just touched with a stick of dry caustic; 
and this was held transversely, so that the tip might be cau- 
terised all round as symmetrically as possible. The radicles 
were then suspended in a closed vessel over water, kept rather 
cool, viz., 55°-59° F. This was done because we had found 
that the tips were more sensitive to contact under a low than 
under a high temperature ; and we thought that the same rule 
might apply to geotropism. In one exceptional trial, nine 
radicles (which were rather too old, for they had grown to a 
length of from 3 to 5 cm.), were extended horizontally in damp 
friable earth, after their tips had been cauterised, and were 
kept at too high a temperature, viz., of 68 ' F., or 20° C. The 
result in consequence was not so striking as in the subsequent 
cases; for although when after 9 h. 40 m. six of them were 
examined, these did not exhibit any geotropic bending, yet after 
24 h., when all nine were examined, only two remained hori- 
zontal, two exhibited a trace of geotropism, and five were 
slightly or moderately geotropic, yet not comparable in degree 
with the control specimens. Marks had been made on seven of 
these cauterised radicles at 10 mm. from the tips, which includes 



Ohai\ XL TRANSMITTED EFFECTS : VICIA. 



551 



the whole growing portion ; and after the 24 h. this part had 
a mean length of 37 mm., so that it had increased to more 
than 3.j times its original length; but it should be remembered 
that these beans had been exposed to a rather high tempeiatare. 
Nineteen young radicles with cauterised tips were extended 
at different times horizontally over water. In over} trial an 
equal number of control specimens were observed. In the first 
trial, the tips of three radicles were lightly touched with the 
caustic for 6 or 7 seconds, which was a longer application than 
usual. After 23 h. 30 m. (temp. 55°-56° F.) these three radicles 




Vicia faba . state of radicles which had been extended horizontally for 
23 h. 30 m. : A, B, C, tips touched with caustic ; D, E, F, tips uncaute- 
rised. Lengths of radicles reduced to one-half scale, but by an accident 
the beans themselves not reduced in the same degree. 



A, B, C (Fig 196), were still horizontal, whilst the three control 
specimens had become within 8 h. slightly geoh\>pic, and 
strongly so {D, E, F) in 23 h. 30 m. A dot had been made on 
all six radicles at 10 mm. from their tips, when first placed 
horizontally. After the 23 h. 30 m. this terminal part, originally 
10 mm. in length, had increased in the cauterised specimens to 
a mean length of 17*3 mm., and to 15*7 mm. in the control 
radicles, as shown in the figures by the unbroken transverse 
line ; the dotted line being at 10 mm. from the apex. The con- 
trol or uncaucerised radicles, therefore, had actually grown Lass 



532 SENSITIVENESS TO GKAVITATION. Chap. XI 

than the canterised ; but this no doubt was accidental, for 
radicles of different ages grow at different rates, and the growth 
of different individuals is likewise affected by unknown causes. 
The state of the tips of these three radicles, which had been 
cauterised for a rather longer time than usual, was as fallows : 
the blackened apex, or the part which had been actually touched 
by the caustic, was succeeded by a yellowish zone, due probably 
to the absorption of some of the caustic; in A, both zones 
together were 11 mm. in length, and 1*4 mm. in diameter at the 
base of the yellowish zone; in B, the length of both was only 
7 mm., and the diameter 0'7 mm.; in C, the length was 0*8 
mm., and the diameter 12 mm. 

Three other radicles, the tips of which had been touched with 
caustic during 2 or 3 seconds, remained (temp. 58°-59° F ) 
horizontal for 23 h. ; the control radicles having, of course, 
become geotropic within this time. The terminal growing part, 
10 mm. in length, of the cauterised radicles had increased in 
this interval to a mean length of 24*5 mm., and of the controls 
to a mean of 26 mm. A section of one of the cauterised tips 
showed that the blackened part was 0*5 mm. in length, of which 
0"2mm. extended into the vegetative point; and a faint dis- 
coloration could be detected even to 1 ■ 6 ram. from the apex of 
the root-cap. 

In another lot of six radicles (temp. 55°- 57° F.) the three 
control specimens were plainly geotropic in 81 h. ; and after 24 h. 
the mean length of their terminal part had increased from 
10 mm. to 21 mm. When the caustic was applied to the three 
cauterised specimens, it was held quite motionless during 

5 seconds, and the result was that the black marks were ex- 
tremely minute. Therefore, caustic was again applied, after 
81 h., during which time no geotropic action had occurred. 
When the specimens were re-examined after an additional 
interval of 151 h., one was horizontal and the other two showed, 
to our surprise, a trace of geotropism which in one of them 
soon afterwards became strongly marked; but in this latter 
specimen the discoloured tip was only § mm. in length. The 
growing part of these three radicles increased in 21 h. from 
10 mm. to an average of 16*5 mm. 

It would be superfluous to describe in detail the 1 e^aviour 
of the 10 remaining cauterised radicles. The corresponding 
control specimens all became geotropic in 8 h. Of the cauterised; 

6 were first looked at after 8 h., and one alone showed a tracf 



Chap. XI. TRANSMITTED EFFECTS : VICLV. 533 

of geotropism ; 4 were first looked at after 14 h., and one alone 
of these was slightly geotropic. After 23-21 h., 5 of the 10 were 
still horizontal, 4 slightly, and 1 decidedly, geotropic. After 
48 h. some of them became strongly geotropic. The cauterised 
radicles increased greatly in length, but the measurements are 
not worth giving. 

As five of the last-mentioned cauterised radicles had become in 
24 h. somewhat geotropic, these (together with three which were 
still horizontal) had their positions reversed, so that their tips 
were now a little upturned; and they were again touched with 
caustic. After 24 h. they showed no trace of geotropism ; whereas 
the eight corresponding control specimens, which had like- 
wise been reversed, in which position the tips of several pointed 
to the zenith, all became geotropic; some having passed in the 
24 h. through an angle of 180°, others through about 13-3°, and 
others through only 90°. The eight radicles, which had been 
twice cauterised, were observed for an additional day (i.e. for 48 h. 
after being reversed), and they still showed no signs of geotro- 
pism. Nevertheless, they continued to gr >w rapidly ; four were 
measured 24 h. after being reversed, and they had in this time 
increased in length betweon 8 and 11 mm. ; the other four were 
measured 48 h. after being reversed, and these had increased by 
20, 18, 23, and 28 mm. 

In coming to a conclusion with respect to the effects of cauter- 
ising the tips of these radicles, we should bear in mind, 
firstly, that horizontally extended control radicles were always 
acted on by geotropism, and became somewhat bowed down- 
wards in 8 or 9 h. ; secondly, that the chief seat of the curvature 
lies at a distance of from 8 to 6 mm. from the tip ; thirdly, that 
the tip was discoloured by the caustic rarely for more than 
1 mm. in length ; fourthly, that the greater number of the cau- 
terised radicles, although subjected to the full influence of 
geotropism during the whole time, remained horizontal for 24 h., 
and some for twice as long ; and that those which did become 
bowed were so only in a slight degree ; fifthly, that the cau- 
terised radicles continued to grow almost, and sometimes quite, 
as well as the uninjured ones along the part which bends most. 
And lastly, that a touch on the tip with caustic, if on one side, 
far from preventing curvature, actually induces it. Bearing all 
these facts in mind, we must infer that under normal conditions 
the geotropic curvature of the root is due to an influence trans- 
mitted from the apex to the adjoining part where the bending 



534 SENSITIVENESS TO GKAVITATION. Chap. XL 

takes place ; and that when the tip of the root is cauterised it is 
unable to originate the stimulus necessary to produce geotropic 
curvature. 

As we had observed that grease was highly injurious to some 
plants, we determined to try its effects on radicles. When the 
cotyledons of Phalaris and Avena were covered with grease 
along one side, the growth of this side was quite stopped or 
greatly checked, and as the opposite side continued to grow, the 
cotyledons thus treated became bowed towards the greased side, 
This same matter quickly killed the delicate hypocotyls and 
young leaves of certain plants. The grease which we employed 
was made by mixing lamp-black and olive oil to such a con- 
sistence that it could be laid on in a thick layer. The tips of 
five radicles of the bean were coated with it for a length of 
3 mm., and to our surprise this part increased in length in 23 h. 
to 7"1 mm.; the thick layer of grease being curiously drawn 
out. It thus could not have checked much, if at all, the growth 
of the terminal part of the radicle. With respect to geotropism, 
the tips of seven horizontally extended radicles were coated for 
a length of 2 mm., and after 24 h. no clear difference could be 
perceived between their downward curvature and that of an 
equal number of control specimens. The tips of 33 other radicles 
were coated on different occasions for a length of 3 mm. ; and 
they were compared with the controls after 8h., 24 h., and 48 h. 
On one occasion, after 24 h., there was very little difference in 
curvature between the greased and control specimens ; but 
generally the difference was unmistakable, those with greased 
tips being considerably less curved downwards. The whole 
growing part (the greased tips included) of six of these radicles 
was measured and was found to have increased in 23 h. from 
10 mm. to a mean length of 17 "7 mm. ; whilst the corresponding 
part of the control had increased to 208 mm. It appears there- 
fore, that although the tip itself, when greased, continues to 
grow, yet the growth of the whole radicle is somewhat checked, 
and that the geotropic curvature of the upper part, which was 
free from grease, was in most cases considerably lessened. 

Pisum sativum. — Five radicles, extended horizontally over 
water, had their tips lightly touched two or three times with dry 
caustic. These tips were measured in two cases, and found to 
be blackened for a length of only half a millimeter. Five other 
radicles were left as controls. The part which is most bowed 
through geotropism lies at a distance of several millimeters from 



Chip. XI. TRANSMITTED EFFECTS : PHASEOLUS. 535 

the apex. After 21 h., and again after 32 h. from the commence- 
ment, four of the cauterised radicles were still horizontal, but 
oue was p'ainly geotropic, being inclined at 45° beneath the 
horizon. The five controls were somewhat geotropic after 7 h. 
2l»m., and after 21 h. were all strongly geotropic ; being inclinod 
at the following angles beneath the horizon, viz., 59°, 60°, 65°, 
57°, and 13°. The length of the radicles was not measured in 
either set, but it was manifest that the cauterised radicles had 
grown greatly. 

The following case proves that the action of the caustic by 
itself does not prevent the curvature of the radicle. Ten radicles 
were extended horizontally on and beneath a layer of damp 
friable peat-earth; and before being extended their tips were 
touched with dry caustic on the upper side. Ten other radicles 
similarly placed were touched on the lower side ; and this would 
tend to make them bend from the cauterised side ; and therefore, 
as now placed, upwards, or in opposition to geotropism. Lastly, 
ten uncauterisecl radicles were extended horizontally as controls. 
After 24 h. all the latter were geotropic ; and the ten with their 
tips cauterised on the upper side were equally geotropic; and 
we believe that they became curved downwards before the con- 
trols. The teu which had been cauterised on the lower side 
presented a widely different appearance : No. 1, however, was 
perpendicularly geotropic, but this was no real exception, for on 
examination under the microscope, there was no vestige of 
a coloured mark on the tip, and it was evident that by a mistake 
it had not been touched with the caustic. No. 2 was plainly 
geotropic, being inclined at about 45° beneath the horizon; No. 3 
was slightly, and No. 4 only just perceptibly geotropic ; Nos. 5 
and 6 were strictly horizontal ; and the four remaining ones were 
bowed upwards, in opposition to geotropism. In these four 
cases the radius of the upward curvatures (according to Sachs' 
cyclometer) was 5 mm., 10 mm., 30 mm., and 70 mm. This cur- 
vature was distinct long before the 24 h. had elapsed, namely, 
after 8 h. 45 m. from the time when the lower sides of the tips 
were touched with the caustic. 

Phaseolu* niultiflorns. — Eight radicles, serving as controls, were 
extended horizontally, some in damp friable peat and some in 
damp air. They a : l became (temp. 1 ; 0°-21° C.) plainly geo- 
tropic in 8 h. 30 in., for they then stood at an average angle of 63 c 
beneath the horizon. A rather greater length of the radicle it 
bowed downwards bj geotropism than in the case of Vicia fubn 
35 



D36 SENSITIVENESS TO GRAVITATION. Chap. XI. 

that is to say, rather more than 6 mm. as measured from the apex 
of the root-cap. Nine other radicles were similarly extended, 
three in damp peat and six in damp air, and dry caustic was 
held transversely to their tips during 4 or 5 seconds. Three of 
their tips were afterwards examined : in (1) a length of • 68 mm. 
was discoloured, of which the basal • 136 mm. was yellow, the 
apical part being black ; in (2) the discoloration was ■ 65 mm, 
in length, of which the basal • 04 mm. was yellow ; in (3) the dis- 
coloration was 0*6 mm. in length, of which the basal 0*13 mm. 
was yellow. Therefore less than 1 mm. was affected by the caustic, 
but this sufficed almost wholly to prevent geotropic action ; for 
after 24 h. one alone of the nine cauterised radicles became 
slightly geotropic, being now inclined at 10° beneath the horizon ; 
the eight others remained horizontal, though one was curved a 
little laterally. 

The terminal part (10 mm. in length) of the six cauterised 
radicles in the damp air, had more than doubled in length in 
the 24 h., for this part was now on an average 20' 7 mm. long. 
The increase in length within the same time was greater in 
the control specimens, for the terminal part had grown on an 
average from 10 mm. to 26*6 mm. But as the cauterised 
radicles had more than doubled their length in the 24 h., it is 
manifest that they had not been seriously injured by the 
caustic. We may here add that when experimenting on the 
effects of touching one side of the tip with caustic, too much 
was applied at first, and the whole tip (but we believe not more 
than 1 mm. in length) of six horizontally extended radicles was 
killed, and these continued for two or three days to grow out 
horizontally. 

Many trials were made, by coating the tips of horizontally 
extended radicles with the before described thick grease. The 
geotropic curvature of 12 radicles, which were thus coated for 
a length of 2 mm., was delayed during the first 8 or 9 h., but 
after 24 h. was nearly as great as that of the control speci- 
mens. The tips of nine radicles were coated for a length of 3 mm., 
and after 7 h. 10 m. these stood at an average angle of 30° 
beneath the horizon, whilst the controls stood at an average of 
54°. After 24 h. the two lots differed but little in their degree 
of curvature. In some other trials, however, there was a fairly 
well-marked difference after 24 h. between those with greased 
tips and the controls. The terminal part of eight control speci- 
mens increased in 24 h. from 10 mm. to a mean length of 



Chap. XL TRANSMITTED EFFECTS ; CUCURBITA. 537 

24*3 mm., whilst the mean increase of those with greased tips 
was 20*7 mm. The grease, therefore, slightly checked the 
growth of the terminal part, but this part was not much 
injured; for several radicles which had been greased for a 
length of 2 mm. continued to grow during seven days, and were 
then only a little shorter than the controls. The appearance 
presented by these radicles after the seven days was very 
curious, for the black grease had been drawn out into the finest 
longitudinal striae, with dots and reticulations, which covered 
their surfaces for a length of from 26 to 44 mm., or of 1 to 
1*7 inch. We may therefore conclude that grease on the tips 
of the radicles of this Phaseolus somewhat delays and lessens 
the geotropic curvature of the part which ought to bend 
most. 

Gossypium Jierbaceum. — The radicles of this plant bend, 
through the action of geotropism, for a length of about 6 mm. 
Five radicles, placed horizontally in damp air, had their tips 
touched with caustic, and the discoloration extended for a 
length of from f to 1 mm. They showed, after 7 h. 45 m. and 
again after 23 h., not a trace of geotropism ; yet the terminal 
portion, 9 mm. in length, had increased on an average to 
15*9 mm. Six control radicles, after 7 h. 45 m., were all plainly 
geotropic, two of them being vertically dependent, and after 
23 h. all were vertical, or nearly so. 

Cucurbita ovifera. — A large number of trials proved almost 
useless, from the three following causes: Firstly, the tips of 
radicles which have grown somewhat old are only feebly geo- 
tropic if kept in damp air; nor did we succeed well in our 
experiments, until the germinating seeds were placed in peat 
and kept at a rather high temperature. Secondly, the hypocotyls 
of the seeds which were pinned to the lids of the jars gradually 
became arched ; and, as the cotyledons were fixed, the movement 
of the hypocotyl affected the position of the radicle, and caused 
confusion. Thirdly, the point of the radicle is so fine that it is 
difficult not to cauterise it either too much or too little. But 
we managed generally to overcome this latter difficulty, as the 
following experiments show^ which are given to prove that a 
touch with caustic on one side of the tip does not prevent the 
upper part of the radicle from bending. Ten radicles were laid 
horizontally beneath and on damp friable peat, and their tips 
were touched with caustic on the upper side. After 8 h. all 
were plainly geotropic, three of them rectangularly ; after 19 h. 



f 38 SENSITIVENESS TO GRAVITATION. Chap. XI. 

all were strongly gee-tropic, most of them pointing perpen- 
dicularly downwards. Ten other radicles, similarly placed, had 
their tips touched with caustic on the lower side; after 8 h. 
three were slightly geotropic, but not nearly so much so as the 
least geotropic of the foregoing specimens ; four remained hori- 
zontal; and three were curved upwards in opposition to geo- 
tropism. After 19 h. the three which were slightly geotropic 
had become strongly so. Of the four horizontal radicles, one 
alone showed a trace of geotropism; of the three up-curved 
radicles, one retained this curvature, and the other two had 
become horizontal. 

The radicles of this plant, as already remarked, do not succeed 
well in damp air, but the result of one trial may be briefly 
given. Nine young radicles between '3 and *5 inch in length., 
with their tips cauterised and blackened for a length never 
exceeding £ mm., together with eight control specimens, were 
extended horizontally in damp air. After an interval of only 

4 h. 10 m. all the controls were slightly geotropic, whilst not 
one of the cauterised specimens exhibited a trace of this action. 
After 8 h. 35 m., there was the same difference between the 
two sets, but rather more strongly marked. By this time both 
sets had increased greatly in length. The controls, however, 
never became much more curved downwards ; and after 24 h. 
there was no great difference between the two sets in their 
degree of curvature. 

Eight young radicles of nearly equal length (average *36 inch) 
were placed beneath and on peat-earth, and were exposed to a 
temp, of 75°-76° E. Their tips had been touched transversely 
with caustic, and five of them were blackened for a length of 
about 05 mm., whilst the other three were only just visibly dis- 
coloured. In the same box there were 15 control radicles, mostly 
about -36 inch in length, but some rather longer and older, and 
therefore less sensitive. After 5 h., the 15 control radicles were 
all more or less geotropic : after 9 h., eight of them were bent 
down beneath the horizon at various angles between 45° and 90°, 
the remaining seven being only slightly geotropic : after 25 h. all 
were rectangularly geotropic. The state of the eight cauterised 
radicles after the same intervals of time was as follows: after 

5 h. one alone was slightly geotropic, and this was one with 
the tip only a very little discoloured : after 9 h. the one just 
mentioned was rectangularly geotropic, and two others were 
slightly so, and these were the three which had been scarcely 



Chap. XL TKANSMITTED EFFECTS : ZEA. 539 

affected by the caustic; the other five were still strictly hori- 
zontal. After 24 h. 40 m. the three with only slightly discoloured 
tips were bent down rectangularly; the other five were not in 
the least affected, but several of them had grown rather tor- 
tuously, though still in a horizontal plane. The eight cauterised 
radicles which had at first a mean length of - 36 inch, after 9 h. 
had increased to a mean length of "79 inch; and after 24 h. 
40 m. to the extraordinary mean length of 2 inches. There 
was no plain difference in length between the five well cau- 
terised radicles which remained horizontal, and the three with 
slightly cauterised tips which had become abruptly bent down. 
A few of the control radicles were measured after 25 h., and 
they were on an average only a little longer than the cauterised, 
viz., 2*19 inches. We thus see that killing the extreme tip of 
the radicle of this plant for a length of about 0*5 mm., though it 
stops the geotropic bending of the upper part, hardly interferes 
with the growth of the whole radicle. 

In the same box with the 15 control specimens, the rapid geo- 
tropic bending and growth of which have just been described, 
there were six radicles, about *6 inch in length, extended hori- 
zontally, from which the tips had been cut off in a transverse 
direction for a length of barely 1 mm. These radicles were 
examined after 9 h. and again after 24 h. 40 m., and they all 
remained horizontal. They had not become nearly so tortuous 
as those above described which had been cauterised. The 
radicles with their tips cut off had grown in the 24 h. 40 m. as 
much, judging by the eye, as the cauterised specimens. 

Zea mays. — The tips of several radicles, extended horizontally 
in damp air, were dried with blotting-paper and then touched 
in the first trial during 2 or 3 seconds with dry caustic ; but 
this was too long a contact, for the tips were blackened for a 
length of rather above 1 mm. They showed no signs of geo- 
tropism after an interval of 9 h., and were then thrown away. 
In a second trial the tips of three radicles were touched for a 
shorter time, and were blackened for a length of from 0*5 to 
0'75 mm. : they all remained horizontal for 4 h., but after 8 h. 
30 m. one of them, in which the blackened tip was only 0*5 mm. 
in length, was inclined at 21° beneath the horizon. Six con- 
trol radicles all became slightly geotropic in 4 h., and strongly 
so after 8 h. 30 m., with the chief seat of curvature generally 
between 6 or 7 mm. from the apex. In the cauterised specimens, 
the terminal growing part, 10 mm. in length, increased during 



540 SENSITIVENESS TO GKAVITATION. Chap. XL 

the 8 h. 30 m. to a mean length of 13 mm. ; and in the controls 
to 14 3 mm. 

In a third trial the tips of five radicles (exposed to a temp, 
of 70°-71°) were touched with the caustic only once and very 
slightly ; they were afterwards examined under the microscope, 
and the part which was in any way discoloured was on an 
average '76 mm. in length. After 4 h. 10 m. none were bent ; 
after 5 h. 45 m., and again after 23 h. 30 m., they still remained 
horizontal, excepting one which was now inclined 20° beneath 
the horizon. The terminal part, 10 mm. in length, had in- 
creased greatly in length during the 23 h. 30 m., viz., to an 
average of 26 mm. Four control radicles became slightly geo- 
tropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m. 
Their mean length after the 23 h. 30 m. had increased from 
10 mm. to 31 mm. Therefore *a slight cauterisation of the tip 
checks slightly the growth of the whole radicle, and manifestly 
stops the bending of that part which ought to bend most under 
the influence of geotropism and which still continues to 
increase greatly in length. 



Concluding Remarks. — Abundant evidence has now 
been given, showing that with various plants the tip 
of the radicle is alone sensitive to geotropism; and 
that when thus excited, it causes the adjoining parts 
to bend. The exact length of the sensitive part seems 
to be somewhat variable, depending in part on the age 
of the radicle ; but the destruction of a length of from 
less than 1 to 1*5 mm. (about -eVth of an inch), in the 
several species observed, generally sufficed to prevent 
any part of the radicle from bending within 24 h., or 
even for a longer period. The fact of the tip alone 
-being sensitive is so remarkable a fact, that we will 
here give a brief summary of the foregoing experiments. 
The tips were cut off 29 horizontally extended radicles 
of Vicia faba, and with a few exceptions they did not 
become geotropic in 22 or 23 h., whilst unmutilated 
radicles were always bowed downwards in 8 or 9 h. It 
should be borne in mind that the mere act of cutting 



Chap. XL TRANSMITTED EFFECTS : CONCLUSION. 541 

off the tip of a horizontally extended radicle does not 
prevent the adjoining parts from bending, if the tip 
has been previously exposed for an hour or two to the- 
influence of geotropism. The tip after amputation is 
sometimes completely regenerated in three days ; and 
it is possible that it may be able to transmit an 
impulse to the adjoining parts before its complete 
regeneration. The tips of six radicles of Cucurbita 
ovifera were amputated like those of Vicia faba ; and 
these radicles showed no signs of geotropism in 24 h. ; 
whereas the control specimens were slightly affected 
in 5 h., and strongly in 9 h. 

With plants belonging to six genera, the tips of the 
radicles were touched transversely with dry caustic ; 
and the injury thus caused rarely extended for a greater 
length than 1 mm., and sometimes to a less distance, as 
judged by even the faintest discoloration. We thought 
that this would be a better method of destroying the 
vegetative point than cutting it off ; for we knew, from 
many previous experiments and from some given in 
the present chapter, that a touch with caustic on one 
side of the apex, far from preventing the adjoining 
part from bending, caused it to bend. In all the 
following cases, radicles with uncauterised tips were 
observed at the same time and under similar circum- 
stances, and they became, in almost every instance, 
^plainly bowed downwards in one-half or one-tliird of 
the time during which the cauterised specimens were 
observed. With Vicia faba 19 radicles were cau- 
terised ; 12 remained horizontal during 23-24 h. ; 
6 became slightly and 1 strongly geotropic. Eight of 
these radicles were afterwards reversed, and again 
touched with caustic, and none of them became geo- 
tropic in 24 h., whilst the reversed control specimens 
became strongly bowed downwards within this time. 



542 SENSITIVENESS TO GRAVITATION. Chap. XI. 

With. Pisum sativum, fiVe radicles had their tips touched 
with caustic, aud after 32 h. four were still horizontal. 
The control specimens were slightly geotropic in 
7 h. 20 in., and strongly so in 24 h. The tips of 9 other 
radicles of this plant were touched only on the lower 
side, and 6 of them remained horizontal for 24 h., or 
were upturned in opposition to geotrupism ; 2 were 
slightly, and 1 plainly geotropic. With Phaseolus 
multijiorus, 15 radicles were cauterised, and 8 re- 
mained horizontal for 24 h. ; whereas all the controls 
w r ere plainly geotropic in 8 h. 30 m. Of 5 cauterised 
radicles of Gossypium herbaceum, 4 remained horizontal 
for 23 h. and 1 became slightly geotropic ; 6 control 
radicles were distinctly geotropic in 7 h. 45 m. Five 
radicles of Gucurhita ovifera remained horizontal in 
peat-earth during 25 h., and 9 remained so in clamp 
air during 8J- h. ; whilst the controls became slightly 
geotropic in 4 h. 10 m. The tips of 10 radicals of this 
plant were touched on their lower sides, and 6 of 
them remained horizontal or were upturned after 19 h., 
1 being slightly and 3 strongly geotropic. 

Lastly, the tips of several radicles of Vicia.fdba and 
Phaseolus multijiorus were thickly coated with grease 
for a length of 3 mm. This matter, which is highly 
injurious to most plants, did not kill or stop the growth 
of the tips, and only slightly lessened the rate of 
growth of the whole radicle ; but it generally delayed 
a little the geotropic bending of the upper part. 

The several foregoing cases would tell us nothing, 
if the tip itself was the part which became most 
bent ; but we know that it is a part distant from the 
tip by some millimeters which grows quickest, and 
which, under the influence of geotropism, bends most, 
We have no reason to suppose that this part is injured 
by the death or injury of the tip ; and it is certain 



Chap. XL TRANSMITTED EFFECTS : CONCLUSION. 543 

that after the tip has been destroyed this part goes on 
growing at such a rate, that its length was often doubled 
in a day. We have also seen that the destruction of the 
tip does not prevent the adjoining part from bending, 
il this part has already received some influence from 
the tip. As with horizontally extended radicles, of 
which the tip has been cut off or destroyed, the part 
which ought to bend most remains motionless for 
many hours or clays, although exposed at right angles 
to the full influence of geotropism, we must conclude 
that the tip alone is sensitive to this power, and trans- 
mits some influence or stimulus to the adjoining parts, 
causing them to bend. We have direct evidence of 
such transmission ; for when a radicle was left extended 
horizontally for an hour or an hour and a half, by 
which time the supposed influence will have travelled 
a little distance from the tip, and the tip was then 
cut off, the radicle afterwards became bent, although 
placed perpendicularly. The terminal portions of 
several radicles thus treated continued for some time 
to grow in the direction of their newly-acquired curva- 
ture ; for as they were destitute of tips, they were no 
longer acted on by geotropism. But after three or 
four days when new vegetative points were formed, the 
radicles were again acted on by geotropism, and now 
they curved themselves perpendicularly downwards. 
To see anything of the above kind in the animal 
kingdom, we should have to suppose that an animal 
whilst lying down determined to rise up in some par- 
ticular direction ; and that after its head had been cut 
off, an impulse continued to travel very slowly along 
the nerves to the proper muscles ; so that after several 
hours the headless animal rose up in the predeter- 
mined direction. 

As the tip of the radicle has been found to be tho 



544 SENSITIVENESS TO GEAVIIATIO^S Ck^p. XI 

part which is sensitive to geotropisrn in the members of 
such distinct families as the Leguminosae, Malvaceae, 
Cucurbitacese and Grarnineye, we may infer that this 
character is common to the roots of most seedling 
plants. Whilst a root is penetrating the ground, the 
tip must travel first ; and we can see the advantage of 
its being sensitive to geotropisrn, as it has to deter- 
mine the course of the whole root. Whenever the tip 
is deflected by any subterranean obstacle, it will also 
be an advantage that a considerable length of the root 
should be able to bend, more especially as the tip 
itself grows slowly and bends but little, so that the 
proper downward course may be soon recovered. But 
it appears at first sight immaterial whether this were 
effected by the whole growing part being sensitive to 
geotropisrn, or by an influence transmitted exclusively 
from the tip. We should, however, remember that it 
is the tip which is sensitive to the contact of hard 
objects, causing the radicle to bend away from them, 
thus guiding it along the lines of least resistance in 
the soil. It is again the tip which is alone sensitive, 
at least in some cases, to moisture, causing the 
radicle to bend towards its source. These two kinds 
of sensitiveness conquer for a time the sensitiveness 
to geotropisrn, which, however, ultimately prevails. 
Therefore, the three kinds of sensitiveness must often 
come into antagonism ; first one prevailing, and then 
another ; and it would be an advantage, perhaps a 
necessity, for the interweighing and reconciling of 
these three kinds of sensitiveness, that they should 
be all localised in the same group of cells which have 
to transmit the command to the adjoining parts of 
the radicle, causing it to bend to or from the source of 
irritation. 

Finally, the fact of the tip alone being sensitive to 



Chap. XL TRANSMITTED EFFECTS : CONCLUSION. 545 

the attraction of gravity has an important bearing on 
the theory of geotropisin. Authors seem generally to 
look at the bending of a radicle towards the centre of 
the earth, as the direct result of gravitation, which is 
believed to modify the growth of the upper or lower 
surfaces, in such a manner as to induce curvature in 
the proper direction. But we now know that it is the 
tip alone which is acted on, and that this part trans- 
mits some influence to the adjoining parts, causing 
them to curve downwards. Gravity does not appear 
to act in a more direct manner on a radicle, than it 
does on any lowly organised animal, which moves 
away when it feels some weight or pressure. 



5£6 SUMMARY AND Chap. XII. 



CHAPTEE XII. 

Summary and Concluding Remarks. 

Nature of the circumnutating movement — History of a germinating 
seed — The radicle first protrudes and eircumnutates — its tip 
highly sensitive — Emergence of ihe hypocotyl or of the epicotyl 
from the ground under the form of an arch — Its circumnutation 
and that of the cotyledons — The seedling throws up a leaf-bearing 
stem — The circumnutation of all the parts or organs— Modified 
circumnutation — Epinasty and hyponasty — Movements of climbing 
plants — Nyetitropic movements — Movements excited by light and 
gravitation — Localised sensitiveness — Resemblance between the 
movements of plants and animals — The tip of the radicle acts like 
a brain. 

It may be useful to the reader if we briefly sum up 
the chief conclusions, which, as far as we can judge, 
have been fairly well established by the observations 
given in this volume. All the parts or organs in 
every plant whilst they continue to grow, and some 
parts which are provided with pulvini after they have 
ceased to grow, are continually circumnutating. This 
movement commences even before the young seedling- 
has broken through the ground. The nature of the 
movement and its causes, as far as ascertained, have 
been briefly described in the Introduction. Why 
every part of a plant whilst it is growing, and in some 
cases after growth has ceased, should have its cells 
rendered more turgescent and its cell-walls more 
extensile first on one side and then on another, thus 
inducing circumnutation, is not known. It would 
appear as if the changes in the cells required periods 
of rest. 



Cfap. XII. CONCLUDING REMARKS. 547 

In some cases, as with the hypocotyls of Brassica, 
the leaves of Dionaea and the joints of the Grammes, 
the circunmutating movement when viewed under the 
microscope is seen to consist of innumerable small 
oscillations. The part under observation suddenly 
jerks forwards for a length of '002 to *001 of an inch, 
and then slowly retreats for a part of this distance ; 
after a few seconds it again jerks forwards, but with 
many intermissions. The retreating movement appa- 
rently is due to the elasticity of the resisting tissues. 
How far this oscillatory movement is general we do 
not know, as not many circunmutating plants were 
observed by us under the microscope ; but no such 
movement could be detected in the case of Drosera 
with a 2-inch object-glass which we used. The pheno- 
menon is a remarkable one. The whole hypocotyl 
of a cabbage or the whole leaf of a Dionaea could not 
jerk forwards unless a very large number of cells on 
one side were simultaneously affected. Are we to sup- 
pose that these cells steadily become more and more 
turgescent on one side, until the part suddenly yields 
and bends, inducing what may be called a micro- 
scopically minute earthquake in the plant ; or do the 
cells on one side suddenly become turgescent in an 
intermittent manner ; each forward movement thus 
caused being opposed by the elasticity of the tissues ? 

Circumnutation is of paramount importance in the 
life of every plant ; for it is through its modification 
that many highly beneficial or necessary movements 
have been acquired. When light strikes one side 
of a plant, or light changes into darkness, or when 
gravitation acts on a displaced part, the plant is 
enabled in some unknown manner to increase the 
always varying turgescence of the cells on one side ; 
so that the ordinary circunmutating movement is 



548 SUMMAKY AND Chap. XH 

modified, and the part bends either to or from the 
exciting cause ; or it may occupy a new position, as 
in the so-called sleep of leaves. The influence which 
modifies circumnutation may be transmitted from one 
part to another. Innate or constitutional changes, 
independently of any external agency, often modify 
the circumnutating movements at particular periods 
of the life of the plant. As circumnutation is uni- 
versally present, we can understand how it is that 
movements of the same kind have been developed in 
the most distinct members of the vegetable series. 
But it must not be supposed that all the movements 
of plants arise from modified circumnutation ; for, as 
we shall presently see, there is reason to believe that 
this is not the case. 

Having made these few preliminary remarks, we 
will in imagination take a germinating seed, and con- 
sider the part which the various movements play in 
the life-history of the plant. The first change is the 
protrusion of the radicle, which begins at once to 
circumnutate. This movement is immediately modi- 
fied by the attraction of gravity and rendered geo- 
tropic. The radicle, therefore, supposing the seed to 
be lying on the surface, quickly bends downwards, fol- 
lowing a more or less spiral course, as was seen on the 
smoked glass-plates. Sensitiveness to gravitation re- 
sides in the tip ; and it is the tip which transmits 
some influence to the adjoining parts, causing them 
to bend. As soon as the tip, protected by the root- 
cap, reaches the ground, it penetrates the surface, if 
this be soft or friable ; and the act of penetration is 
apparently aided by the rocking or circumnutating 
movement of the whole end of the radicle. If the sur- 
face is compact, and cannot easily be penetrated, then 



Chap. XII. CONCLUDING EEMARKS. 549 

the seed itself, unless it be a heavy one, is displaced 
or lifted up by the continued growth and elongation 
of the radicle. But in a state of nature seeds often 
get covered with earth or other matter, or fall into 
crevices, &c, and thus a point of resistance is afforded, 
and the tip can more easily penetrate the ground. 
But even with seeds lying loose on the surface there 
is another aid : a multitude of excessively fine hairs 
are emitted from the upper part of the radicle, and 
these attach themselves firmly to stones or other ob- 
jects lying on the surface, and can do so even to glass ; 
and thus the upper part is held down whilst the tip 
presses against and penetrates the ground. The 
attachment of the root-hairs is effected by the lique- 
faction of the outer surface of the cellulose walls, and 
by the subsequent setting hard of the liquefied matter. 
This curious process probably takes place, not for 
the sake of the attachment of the radicles to superficial 
objects, but in order that the hairs may be brought into 
the closest contact with the particles in the soil, by 
which means they can absorb the layer of water sur- 
rounding them, together with any dissolved matter. 

After the tip has penetrated the ground to a little 
depth, the increasing thickness of the radicle, togethei 
with the root-hairs, hold it securely in its place ; and 
now the force exerted by the longitudinal growth ol 
the radicle drives the tip deeper into the ground. 
This force, combined with that due to transverse 
growth, gives to the radicle the power of a wedge. 
Even a growing root of moderate size, such as that 
of a seedling bean, can displace a weight of some 
pounds. It is not probable that the tip when, buried 
in compact earth can actually circumnutate and thus 
aid its downward passage, but the circumnutating 
movement will facilitate the tip entering any lateral 



550 SUMMARY AXD Chap. XII 

or oblique fissure in the earth, or a burrow made by 
an earth-worm or larva; and it is certain that roots 
often run down the old burrows of worms. The tip,, 
however, m endeavouring to circumnutate, will con- 
tinually press against the earth on all sides, and this 
can hardly fail to be of the highest importance to the 
plant ; for we have seen that when little bits of card- 
like paper and of very thin paper were cemented on 
opposite sides of the tip, the whole growing part of 
the radicle was excited to bend away from the side 
bearing the card or more resisting substance, towards 
the side bearing the thin paper. We may therefore 
feel almost sure that when the tip encounters a stone 
or other obstacle in the ground, or even earth more 
compact on one side than the other, the root will bend 
away as much as it can from the obstacle or the more 
resisting earth, and will thus follow with unerring 
skill a line of least resistance. 

The tip is more sensitive to prolonged contact with 
an object than to gravitation when this acts obliquely 
on the radicle, and sometimes even when it acts in the 
most favourable direction at right angles to the radicle. 
The tip was excited by an attached bead of shellac, 
weighing less than 200^ °^ a g rain (0'33 mg.) ; it is 
therefore more sensitive than the most delicate ten- 
dril, namely, that of Passiflora gracilis, which was barely 
acted on by a bit of wire weighing Jg-th of a grain. But 
this degree of sensitiveness is as nothing compared with 
that of the glands of Drosera, for these are excited by 
particles weighing only ysTTo °f a g ram - The sensi- 
tiveness of the tip cannot be accounted for by its 
being covered by a thinner layer of tissue than the 
other parts, for it is protected by the relatively thick 
root-cap. It is remarkable that although the radicle 
bends away, when one side of the tip is slightly touched 



Chap. XII. CONCLUDING REMARKS. 551 

with caustic, yet if the side be much cauterised the 
injury is too great, and the power of transmitting some 
influence to the adjoining parts causing them to bend, 
is lost. Other analogous cases are known to occur. 

After a radicle has been deflected by some obstacle, 
geotropism directs the tip again to grow perpendicu- 
larly downwards ; but geotropism is a feeble power, 
and here, as Sachs has shown, another interesting 
adaptive movement comes into play ; for radicles at 
a distance of a few millimeters from the tip are 
sensitive to prolonged contact in such a manner that 
they bend towards the touching object, instead of from 
it as occurs when an object touches one side of the 
tip. Moreover, the curvature thus caused is abrupt; 
the pressed part alone bending. Even slight pressure 
suffices, such as a bit of card cemented to one side. 
Therefore a radicle, as it passes over the edge of any 
obstacle in the ground, will through the action of geo^ 
tropism press against it ; and this pressure will cause 
the radicle to endeavour to bend abruptly over the 
edge. It will thus recover as quickly as possible its 
normal downward course. 

Kadicles are also sensitive to air which contains 
more moisture on one side than the other, and they 
bend towards its source. It is therefore probable that 
they are in like manner sensitive to dampness in the 
soil. It was ascertained in several cases that this 
sensitiveness resides in the tip, which transmits an 
influence causing the adjoining upper part to bend 
in opposition to geotropism towards the moist object. 
We may therefore infer that roots will be deflected 
from their downward course towards any source oi 
moisture in the soil. 

Again, most or all radicles are slightly sensitive to 
light, and, according to Wiesner, generally bend a little 



5£2 SUMMARY AND Chap XII. 

from it. Whether this can be of any service to them 
is very doubtful, but with seeds germinating on the 
surface it will slightly aid geotropism in directing 
the radicles to the ground.* We ascertained in one 
instance that such sensitiveness resided in the tip, and 
caused the adjoining parts to bend from the light. 
The sub-aerial roots observed by Wiesner were all 
apheliotropic, and this, no doubt, is of use in bringing 
them into contact with trunks of trees or surfaces of 
rock, as is their habit. 

We thus see that with seedling plants the tip of the 
radicle is endowed with diverse kinds Of sensitiveness ; 
and that the tip directs the adjoining growing parts 
to bend to or from the exciting cause, according to the 
needs of the plant. The sides of the radicle are also 
sensitive to contact, but in a widely different manner. 
Gravitation, though a less powerful cause of move- 
ment than the other above specified stimuli, is ever 
present ; so that it ultimately prevails and determines 
the downward growth of the root. 

The primary radicle emits secondary ones which 
project sub-horizontally ; and these were observed in 
one case to circumnutate. Their tips are also sensitive 
to contact, and they are thus excited to bend away 
from any touching object; so that they resemble in 
these respects, as far as they were observed, the 
primary radicles. If displaced they resume, as Sachs 
has shown, their original sub-horizontal position ; and 
this apparently is due to diageotropism. The secondary 
radicles emit tertiary ones, but these, in the case of 
the bean, are not affected by gravitation ; consequently 
they protrude in all directions. Thus the general 



* Dr Karl Kichter. who has in Wien,' 1879, p. 149), states thai 
?speciallvatttn ed to this subject apheliotropisra docs not aid ra- 
'' K. Ak;td. der Wisseusehalten dides in penetiuting the ground. 



Chap. XII CONCLUDING REMARKS. 55S 

arrangement of the three orders of roots is ex silently 
adapted for searching the whole soil for nutriment. 

Sachs has shown that if the tip of the primary- 
radicle is cut off (and the tip will occasionally he 
gnawed off with seedlings in a state of nature) one of 
the secondary radicles grows perpendicularly down- 
wards, in a manner which is analogous to the upward 
growth of a lateral shoot after the amputation of 
the leading shoot. We have seen with radicles of the 
bean that if the primary radicle is merely compressed 
instead of being cut off, so that an excess of sap is 
directed into the secondary radicles, their natural con- 
dition is disturbed and they grow downwards. Other 
analogous facts have been given. As anything which 
disturbs the constitution is apt to lead to reversion, 
that is, to the resumption of a former character, it 
appears probable that when secondary radicles grow 
downwards or lateral shoots upwards, they revert to 
the primary manner of growth proper to radicles and 
shoots. 

With dicotyledonous seeds, after the protrusion of 
the radicle, the hypocotyl breaks through the seed- 
coats ; but if the cotyledons are hypogean, it is the 
epicotyl which breaks forth. These organs are at first 
invariably arched, with the upper part bent back 
parallel to the lower ; and they retain this form until 
they have risen above the ground. In some cases, 
however, it is the petioles of the cotyledons or of the 
first true leaves which break through the seed-coats 
as well as the ground, before any part of the stem 
protrudes ; and then the petioles are almost invariabl v 
arched. We have met with only one exception, and that 
only a partial one, namely, with the petioles of the two 
first leaves of Acanthus candelabrum. With Delphinium 
nudicaule the petioles of the two cotyledons are com- 



554 SUMMARY AND Chap. XII. 

pletely confluent, and they break through the ground 
as an arch ; afterwards the petioles of the successively 
formed early leaves are arched, and they are thus 
enabled to break through the base of the confluent 
petioles of the cotyledons. In the case of Megarrhiza, 
it is the plumule which breaks as an arch through the 
tube formed by the confluence of the cotyledon- 
petioles. With mature plants, the flower-stems and 
the leaves of some few species, and the rachis of 
several ferns, as they emerge separately from the 
ground, are likewise arched. 

The fact of so many different organs in plants of 
many kinds breaking through the ground under the 
form of an arch, shows that this must be in some 
manner highly important to them. According to 
Haberlandt, the tender growing apex is thus saved 
from abrasion, and this is probably the true explana- 
tion. But as both legs of the arch grow, their power 
of breaking through the ground will be much in- 
creased as long as the tip remains within the seed- 
coats and has a point of support. In the case of 
monocotyledons the plumule or cotyledon is rarely 
arched, as far as we have seen ; but this is the case 
with the leaf-like cotyledon of the onion ; and the 
crown of the arch is here strengthened by a special 
protuberance. In the Gramineas the summit of the 
straight, sheath-like cotyledon is developed into a 
hard sharp crest, which evidently serves for breaking- 
through the earth. With dicotyledons the arching of 
the epicotyl or hypocotyl often appears as if it merely 
resulted from the manner in which the parts are 
packed within the seed; but it is doubtful whether 
this is the whole of the truth in any case, and it cer- 
tainly was not so in several cases, in which the arch- 
ing was seen to commence after the parts had wholly 



Chap. XII. CONCLUDING REMARKS. 555 

escaped from the seed-coats. As the arching occurred 
in whatever position the seeds were placed, it is no 
doubt due to temporarily increased growth of the 
nature of epinasty or hyponasty along one side of the 
part. 

As this habit of the hypocotyl to arch itself appears 
to be universal, it is probably of very ancient origin. 
It is therefore not surprising that it should be in- 
herited, at least to some extent, by plants having 
hypogean cotyledons, in which the hypocotyl is only 
slightly developed and never protrudes above the 
ground, and in which the arching is of course now 
quite useless. This tendency explains, as we have 
seen, the curvature of the hypocotyl (and the conse- 
quent movement of the radicle) which was first 
observed by Sachs, and which we have often had to 
refer to as Sachs' curvature. 

The several foregoing arched organs are continually 
circumnutating, or endeavouring to circumnutate, even 
before they break through the ground. As soon as 
any part of the arch protrudes from the seed-coats it 
is acted upon by apogeotropism, and both the legs 
bend upwards as quickly as the surrounding earth will 
permit, until the arch stands vertically. By continued 
growth it then forcibly breaks through the ground ; 
but as it is continually striving to circumnutate this 
will aid its emergence in some slight degree, for we 
know that a circumnutating hypocotyl can push away 
damp sand on all sides. As soon as the faintest ray of 
light reaches a seedling, heliotropism will guide it 
through any crack in the soil, or through an entangled 
mass of overlying vegetation ; for apogeotropism by 
itself can direct the seedling only blindly upwards. 
Hence probably it is that sensitiveness to light resides 
in the tip of the cotyledons of the Grraminese, and in 



556 SUMMARY AND Chap. XII. 

the upper part of the hypocotyls of at least some 
plants. 

As the arch grows upwards the cotyledons are 
dragged out of the ground. The seed-coats are either 
left behind buried, or are retained for a time still 
enclosing the cotyledons. These are afterwards cast 
off merely by the swelling of the cotyledons. But 
with most of the Cucurbitacese there is a curious 
special contrivance for bursting the seed-coats whilst 
beneath the ground, namely, a peg at the base of the 
hypocotyl, projecting at right angles, which holds down 
the lower half of the seed- coats, whilst the growth 
of the arched part of the hypocotyl lifts up the upper 
half, and thus splits them in twain. A somewhat 
analogous structure occurs in Mimosa pudica and some 
other plants. Before the cotyledons are fully ex- 
panded and have diverged, the hypocotyl generally 
straightens itself by increased growth along the con- 
cave side, thus reversing the process which caused 
the arching. Ultimately not a trace of the former 
curvature is left, except in the case of the leaf-like 
cotyledons of the onion. 

The cotyledons can now assume the function of 
leaves, and decompose carbonic acid ; they also yield 
up to other parts of the plant the nutriment which 
they often contain. When they contain a large stock 
of nutriment they generally remain buried beneath 
the ground, owing to the small development of the 
hypocotyl ; and thus they have a better chance of 
escaping destruction by animals. From unknown 
causes, nutriment is sometimes stored in the hypocotyl 
or in the radicle, and then one of the cotyledons or 
both become rudimentary, of which several instances 
have been given. It is probable that the extraordi- 
nary manner of germination of Megarrhiza Calif or niea, 



Chap. XII. CONCLUDING REMARKS. 557 

Ipomoea leptophylla and pandurata, and of Quercus 
virens, is connected with the burying of the tuber-like 
roots, which at an early age are stocked with nutri- 
ment ; for in these plants it is the petioles of the 
cotyledons which first protrude from the seeds, and 
they are then merely tipped with a minute radicle and 
hypocotyl. These petioles bend down geotropically 
like a root and penetrate the ground, so that the true 
root, which afterwards becomes greatly enlarged, is 
buried at some little depth beneath the surface. Gra- 
dations of structure are always interesting, and Asa 
Gray informs us that with Ipomoea Jalappa, which 
likewise forms huge tubers, the hypocotyl is still of 
considerable length, and the petioles of the cotyledons 
are only moderately elongated. But in addition to the 
advantage gained by the concealment of the nutritious 
matter stored within the tubers, the plumule, at least 
in the case of Megarrhiza, is protected from the frosts 
of winter by being buried. 

With many dicotyledonous seedlings, as has lately 
been described by De Yries, the contraction of the 
parenchyma of the upper part of the radicle drags the 
hypocotyl downwards into the earth ; sometimes (it is 
said) until even the cotyledons are buried. The hypo- 
cotyl itself of some species contracts in a like manner. 
It is believed that this burying process serves to 
protect the seedlings against the frosts of winter. 

Our imaginary seedling is now mature as a seedling, 
for its hypocotyl is straight and its cotyledons are 
fully expanded. In this state the upper part of the 
hypocotyl and the cotyledons continue for some time 
to circumnutate, generally to a wide extent relat. vely 
to the size of the parts, and at a rapid rate. But 
seedlings profit by this power of movement only w hen 
it is modified, especially by the action of light and 



558 SUMMARY AND Chap. XII. 

gravitation ; for they are thus enabled to move more 
rapidly and to a greater extent than can most mature 
plants. Seedlings are subjected to a severe struggle 
for life, and it appears to be highly important to them 
that they should adapt themselves as quickly and as 
perfectly as possible to their conditions. Hence also 
it is that they are so extremely sensitive to light and 
gravitation. The cotyledons of some few species are 
sensitive to a touch ; but it is probable that this is 
only an indirect result of the foregoing kinds of sen- 
sitiveness, for there is no reason to believe that they 
profit by moving when touched. 

Our seedling now throws up a stem bearing leaves, 
and often branches, all of which whilst young are con- 
tinually circumnutating. If we look, for instance, at a 
great acacia tree, we may feel assured that every one of 
the innumerable growing shoots is constantly describ- 
ing small ellipses ; as is each petiole, sub-petiole, and 
leaflet. The latter, as well as ordinary leaves, gene- 
rally move up and down in nearly the same vertical 
plane, so that they describe very narrow ellipses. 
The flower-peduncles are likewise continually circum- 
nutating. If we could look beneath the ground, and 
our eyes had the power of a microscope, we should see 
the tip of each rootlet endeavouring to sweep small 
ellipses or circles, as far as the pressure of the sur- 
rounding earth permitted. All this astonishing amount 
of movement has been going on year after year since 
the time when, as a seedling, the tree first emerged 
from the ground. 

Stems are sometimes developed into long runners or 
stolons. These circumnutate in a conspicuous manner, and 
are thus aided in passing between and over surrounding 
obstacles. But whether the circumnutating movement 
has been increased for this special purpose is doubtful. 



Chap. XII. CONCLUDING REMARKS. 55D 

We have now to consider circunmutation in a 
modified form, as the source of several great classes of 
movement. The modification may be determined by- 
innate causes, or by external agencies. Under the first 
head we see leaves which, when first unfolded, stand 
in a vertical position, and gradually bend downwards 
as they grow older. We see flower-peduncles bending 
down after the flower has withered, and others rising 
up ; or again, stems with their tips at first bowed 
downwards, so as to be hooked, afterwards straighten- 
ing themselves ; and many other such cases. These 
changes of position, which are due to epinasty or 
hyponasty, occur at certain periods of the life of the 
plant, and are independent of any external agency. 
They are effected not by a continuous upward or 
downward movement, but by a succession of small 
ellipses, or by zigzag lines, — that is, by a circum- 
nutating movement which is preponderant in some 
one direction. 

Again, climbing plants whilst young circumnutate 
in the ordinary manner, but as soon as the stem 
has grown to a certain height, which is different for 
different species, it elongates rapidly, and now the 
amplitude of the circumnutating movement is im- 
mensely increased, evidently to favour the stem catch- 
ing hold of a support. The stem also circumnutates 
rather more equally to all sides than in the case of 
non-climbing plants. This is conspicuously the case 
with those tendiils which consist of modified leaves, 
as these sweep wide circles ; whilst ordinary leaves 
usually circumnutate nearly in the same vertical plane. 
Flower-peduncles when converted into tendrils have 
their circumnutating movement in like manner greatly 
increased. 

We now come to our second group of circumnu- 



560 SUMMARY AND Chap. XII 

tating movements — those modified through externa] 
agencies. The so-called sleep or nyctitropic move- 
ments of leaves are determined by the daily alterna- 
tions of light and darkness. It is not the darkness 
which excites them to move, but the difference in the 
amount of light which they receive during the day 
and night ; for with several species, if the leaves have 
not been brightly illuminated during the day, they 
do not sleep at night. They inherit, however, some 
tendency to move at the proper periods, indepen- 
dently of any change in the amount of light. The 
movements are in some cases extraordinarily complex, 
but as a full summary has been given in the chapter 
devoted to this subject, we will here say but little on 
this head. Leaves and cotyledons assume their noc- 
turnal position by two means, by the aid of pulvini and 
without such aid. In the former case the movement 
continues as long as the leaf or cotyledon remains in 
full health ; whilst in the latter case it continues only 
whilst the part is growing. Cotyledons appear to 
sleep in a larger proportional number of species than 
do leaves. In some species, the leaves sleep and not 
the cotyledons ; in others, the cotyledons and not the 
leaves ; or both may sleep, and yet assume widely 
different positions at night. 

Although the nyctitropic movements of leaves and 
cotyledons are wonderfully diversified, and sometimes 
differ much in the species of the same genus, yet the 
blade is always placed in such a position at night, that 
its upper surface is exposed as little as possible to full 
radiation. We cannot doubt that this is the object 
gained by these movements ; and it has been proved 
that leaves exposed to a clear sky, with their blades 
compelled to remain horizontal, suffered much more 
from the cold than others svhich were allowed to assume 



Osap XII CONCLUDING REMARKS. 561 

their proper vertical position. Some curious facts 
have heen given under this head, showing that hori- 
zontally extended leaves suffered more at night, when 
the air, which is not cooled by radiation, was prevented 
from freely circulating beneath their lower surfaces ; 
and so it was, when the leaves were allowed to go to 
sleep on branches which had been rendered motionless. 
In some species the petioles rise up greatly at night, 
and the pinnae close together. The whole plant is 
thus rendered more compact, and a much smaller 
surface is exposed to radiation. 

That the various nyctitropic movements of leaves 
result from modified circumnutation has, we think, 
been clearly shown. In the simplest cases a leaf 
describes a single large ellipse during the 24 h. ; and 
the movement is so arranged that the blade stands 
vertically during the night, and reassumes its former 
position on the following morning. The course pursued 
differs from ordinary circumnutation only in its greater 
amplitude, and in its greater rapidity late in the 
evening and early on the following morning. Unless 
this movement is admitted to be one of circumnu- 
tation, such leaA r es do not circumnutate at all, and this 
would be a monstrous anomaly. In other cases, leaves 
and cotyledons describe several vertical ellipses during 
the 24 h. ; and in the evening one of them is increased 
greatly in amplitude until the blade stands vertically 
either upwards or downwards. In this position it con- 
tinues to circumnutate until the following morning, 
when it reassumes its former position. These move- 
ments, when a pulvinus is present, are often compli- 
cated by the rotation of the leaf or leaflet ; and such 
rotation on a small scale occurs during ordinary cir- 
cumnutation. The many diagrams showing the move- 
ments of sleeping and non-sleeping leaves and coty- 



562 SUMMARY AND Chap. XII 

ledons should be compared, and it will be seen that 
they are essentially alike. Ordinary circumnntation 
is converted into a nyctitropic movement, firstly by an 
increase in its amplitude, but not to so great a degree 
as in the case of climbing plants, and secondly by its 
being rendered periodic in relation to the alterna- 
tions of day and night. But there is frequently a 
distinct trace of periodicity in the circumnutating 
movements of non-sleeping leaves and cotyledons. 
The fact that nyctitropic movements occur in species 
distributed in many families throughout the whole 
vascular series, is intelligible, if they result from the 
modification of the universally present movement of 
circumnutation ; otherwise the fact is inexplicable. 

In the seventh chapter we have given the case of 
a Porlieria, the leaflets of which remained closed all 
day, as if asleep, when the plant was kept dry, appa- 
rently for the sake of checking evaporation. Some- 
thing of the same kind occurs with certain G-raminese. 
At the close of this same ' chapter, a few observations 
were appended on what may be called the embryology 
of leaves. The leaves produced by young shoots on 
cut-down plants of Melilotus taurica slept like those of 
a Trifolium, whilst the leaves on the older branches 
on the same plants slept in a very different manner, 
proper to the genus ; and from the reasons assigned 
we are tempted to look at this case as one of reversion 
to a former nyctitropic habit. So again with Desmo- 
dium gyrans, the absence of small lateral leaflets on 
very young plants, makes us suspect that the imme- 
diate progenitor of this species did not possess lateral 
leaflets, and that their appearance in an almost rudi- 
mentary condition at a somewhat more advanced age 
is the result of reversion to a trifoliate predecessor. 
However this may be, the rapid circumnutating or 



Chap. XIL CONCLUDING REMARKS. 563 

gyrating movements of the little lateral leaflets, seem 
to be due proximately to the pulvinus, or organ of 
movement, not having been reduced nearly so much 
as the blade, during the successive modifications 
through which the species has passed. 

We now come to the highly important class of 
movements due to the action of a lateral light. When 
stems, leaves, or other organs are placed, so that one 
side is illuminated more brightly than the other, they 
bend towards the light. This heliotropic movement 
manifestly results from the modification of ordinary 
circumnutation ; and every gradation between the two 
movements could be followed. When the light was 
dim, and only a very little brighter on one side than 
on the other, the movement consisted of a succession 
of ellipses, directed towards the light, each of which 
approached nearer to its source than the previous one. 
When the difference in the light on the two sides 
was somewhat greater, the ellipses were drawn out 
into a strongly-marked zigzag line, and when much 
greater the course became rectilinear. We have 
reason to believe that changes in the turgescence of 
the cells is the proximate cause of the movement 
of circumnutation ; and it appears that when a plant 
is unequally illuminated on the two sides, the always 
changing turgescence is augmented along one side, 
and is weakened or quite arrested along the other 
sides. Increased turgescence is commonly followed by 
increased growth, so that a plant which has bent itself 
towards the light during the day would be fixed in this 
position were it not for apogeotropism acting during 
the night. But parts provided with pulvini bend, as 
Pfeffer has shown, towards the light ; and here growth 
does not come into play any more than in the ordinary 
circumnutating movements of pulvini. 



564 SUMMARY AND Chap. XII. 

Heliotropism prevails widely throughout the vege- 
table kingdom, but whenever, from the changed habits 
of life of any plant, such movements become injurious 
or useless, the tendency is easily eliminated, as we see 
with climbing and insectivorous plants. 

Apheliotropic movements are comparatively rare in 
a well-marked degree, excepting with sub-aerial roots. 
In the two cases investigated by us, the movement 
certainly consisted of modified circumnutation. 

The position which leaves and cotyledons occupy 
during the day, namely, more or less transversely to 
the direction of the light, is due, according to Frank, 
to what we call diaheliotropism. As all leaves and 
cotyledons are continually circumnutating, there can 
hardly be a doubt that diaheliotropism results from 
modified circumnutation. From the fact of leaves and 
cotyledons frequently rising a little in the evening, it 
appears as if diaheliotropism had to conquer during 
the middle of the day a widely prevalent tendency to 
apogeotropism. 

Lastly, the leaflets and cotyledons of some plants 
are known to be injured by too much light ; and when 
the sun shines brightly on them, they move upwards 
or downwards, or twist laterally, so that they direct 
their edges towards the light, and thus they escape 
being injured. These paraheliotropic movements cer- 
tainly consisted in one case of modified circumnuta- 
tion ; and so it probably is in all cases, for the leaves 
of all the species described circumnutate in a con- 
spicuous manner. This movement has hitherto been 
observed only with leaflets provided with pulvini, in 
which the increased turgescence on opposite sides is 
not followed by growth ; and we can understand why 
this should be so, as the movement is required only 
for a temporary purpose. It would manifestly be dis« 



Chap. XIL CONCLUDING REMAKES. 565 

advantageous for the leaf to be fixed by growth in its 
inclined position. For it has to assume its former 
horizontal position, as soon as possible after the sun 
has ceased shining too brightly on it. 

The extreme sensitiveness of certain seedlings to 
light, as shown in our ninth chapter, is highly remark- 
able. The cotyledons of Phalaris became curved 
towards a distant lamp, which emitted so little light, 
that a pencil held vertically close to the plants, did 
not cast any shadow which the eye could perceive 
on a white card. These cotyledons, therefore, were 
affected by a difference in he amount of light on their 
two sides, which the eye could not distinguish. The 
degree of their curvature within a given time towards 
a lateral light did not correspond at all strictly with 
the amount of light which they received ; the light 
not being at any time in excess. They continued for 
nearly half an hour to bend towards a lateral light, 
after it had been extinguished. They bend with 
remarkable precision towards it, and this depends on 
the illumination of one whole side, or on the obscura- 
tion of the whole opposite side. The difference in the 
amount of light which plants at any time receive in 
comparison with what they have shortly before re- 
ceived, seems in all cases to be the chief exciting cause 
of those movements which are influenced by light. 
Thus seedlings brought out of darkness bend towards 
a dim lateral light, sooner than others which had pre- 
viously been exposed to daylight. We have seen 
several analogous cases with the nyctitropic move- 
ments of leaves. A striking instance was observed in 
the case of the periodic movements of the cotyledons 
of a Cassia; in the morning a pot was placed in an 
obscure part of a room, and all the cotyledons rose up 
closed • anot her pot had stood in the sunlight, and 



566 SUMMAEY AND Chap. XII. 

the cotyledons of course remained expanded; both 
pots were now placed close together in the middle oi 
the room, and the cotyledons which had been exposed 
to the sun, immediately began to close, while the 
others opened ; so that the cotyledons in the two pots 
moved in exactly opposite directions whilst exposed 
to the same degree of light. 

We found that if seedlings, kept in a dark place, 
were laterally illuminated by a small wax taper for 
only two or three minutes at intervals of about three- 
quarters of an hour, they all became bowed to the 
point where the taper had been held. We felt much 
surprised at this fact, and until we had read Wiesner's 
observations, we attributed it to the after-effects of 
the light ; but he has shown that the same degree 
of curvature in a plant may be induced in the 
course of an hour by several interrupted illumina- 
tions lasting altogether for 20 m., as by a continuous 
illumination of 60 m. We believe that this case, 
as well as our own, may be explained by the ex- 
citement from light being due not so much to its 
actual amount, as to the difference in amount from 
that previously received; and in our case there were 
repeated alternations from complete darkness to light. 
In this, and in several of the above specified respects, 
light seems to act on the tissues of plants, almost in 
the same manner as it does on the nervous system 
of animals. 

There is a much more striking analogy of the same 
kind, in the sensitiveness to light being localised in 
the tips of the cotyledons of Phalaris and Avena, and 
in the upper part of the hypocotyls of Brassica and 
Beta ; and in the transmission of some influence from 
these upper to the lower parts, causing the latter to 
bend towards the light. This influence is also trans- 



Chap. XII. CONCLUDING REMARKS. 567 

mittecl beneath the soil to a depth where no light 
enters. It follows from this localisation, that the 
lower parts of the cotyledons of Phalaris, &c, which 
normally become more bent towards a lateral light 
than the upper parts, may be brightly illuminated 
during many hours, and will not bend in the least, if 
all light be excluded from the tip. It is an interest- 
ing experiment to place caps over the tips of the 
cotyledons of Phalaris, and to allow a very little light 
to enter through minute orifices on one side of the 
caps, for the lower part of the cotyledons will then 
bend to this side, and not to the side which has been 
brightly illuminated during the whole time. In the 
case of the radicles of Sinajpis alba, sensitiveness to 
light also resides in the tip, which, when laterally 
illuminated, causes the adjoining part of the root to 
bend apheliotropically. 

Gravitation excites plants to bend away from the 
centre of the earth, or towards it, or to place them- 
selves in a transverse position with respect to it. 
Although it is impossible to modify in any direct 
manner the attraction of gravity, yet its influence 
could be moderated indirectly, in the several ways 
described in the tenth chapter; and under such 
circumstances the same kind of evidence as that given 
in the chapter on Heliotropism, showed in the plainest 
manner that apogeotropic and geotropic, and probably 
diageotropic movements, are all modified forms of 
circumnutation. 

Different parts of the same plant and different 
species are affected by gravitation in widely different 
degrees and manners. Some plants and organs exhibit 
hardly a trace of its action. Young seedlings which, 
as we know, circumnutate rapidly, are eminently sensi- 
tive ; and we have seen the hypocotyl of Beta bending 
37 



$68 SUMMAEY AND Chap. XIL 

upwards through 109° in 3 h. 8 m. The after-effects 
of apogeotropism last for above half an hour ; and 
horizontally-laid hypocotyls are sometimes thus car- 
ried temporarily beyond an upright position. The 
benefits derived from geotropism, apogeotropism, and 
diageotropism, are generally so manifest that they 
need not be specified. With the flower-peduncles of 
Oxalis, epinasty causes them to bend down, so that 
the ripening pods may be protected by the calyx 
from the rain. Afterwards they are carried upwards 
by apogeotropism in combination with hyponasty, and 
are thus enabled to scatter their seeds over a wider 
space. The capsules and flower-heads of some plants 
are bowed downwards through geotropism, and they 
then bury themselves in the earth for the protection 
and slow maturation of the seeds. This burying 
process is much facilitated by the rocking movement 
due to circumnutation. 

In the case of the radicles of several, probably of all 
seedling plants, sensitiveness to gravitation is confined 
to the tip, which transmits an influence to the adjoining 
upper part, causing it to bend towards the centre of 
the earth. That there is transmission of this kind was 
proved in an interesting manner when horizontally 
extended radicles of the bean were exposed to the 
attraction of gravity for 1 or 1 J h., and their tips were 
then amputated. Within this time no trace of curva- 
ture was exhibited, and the radicles were now placed 
pointing vertically downwards ; but an influence had 
already been transmitted from the tip to the adjoining 
part, for it soon became bent to one side, in the same 
manner as would have occurred had the radicle 
remained horizontal and been still acted on by geo- 
tropism. Kadicles thus treated continued to grow out 
horizontally for two or three days, until a new tip was 



Chap. XII. CONCLUDING REMARKS. 569 

reformed ; and this was then acted on by geotropism, 
and the radicle became curved perpendicularly down 
wards. 

It has now been shown that the following important 
classes of movement all arise from modified circum- 
nutation, which is omnipresent whilst growth lasts, 
and after growth has ceased, whenever pulvini are 
present. These classes of rnovement consist of those 
due to epinasty and hyponasty, — those proper to 
climbing plants, commonly called revolving nutation, 
— the nyctitropic or sleep movements of leaves and 
cotyledons, — and the two immense classes of move- 
ment excited by light and gravitation. When we 
speak of modified circumnutation we mean that light, 
or the alternations of light and darkness, gravitation, 
slight pressure or other irritants, and certain innate 
or constitutional states of the plant, do not directly 
cause the movement ; they merely lead to a tempo- 
rary increase or diminution of those spontaneous 
changes in the turgescence of the cells which are 
already in progress. In what manner, light, gravita- 
tion, &c, act on the cells is not known ; and we 
will here only remark that, if any stimulus affected 
the cells in such a manner as to cause some slight 
tendency in the affected part to bend in a beneficial 
manner, this tendency might easily be increased 
through the preservation of the more sensitive indi- 
viduals. But if such bending were injurious, the 
tendency would be eliminated unless it was over- 
poweringly strong; for we know how commonly all 
characters in all organisms vary. Nor can we see any 
reason to doubt, that after the complete elimination of 
a tendency to bend in some one direction under a 
certain stimulus, the power to bend in a directly 



570 SUMMARY AND, Chap. XII 

opposite direction might gradually be acquired through 
natural selection.* 

Although so many movements have arisen through 
modified circumnutation, there are others which 
appear to have had a quite independent origin ; but 
they do not form such large and important classes. 
When a leaf of a Mimosa is touched it suddenly 
assumes the same position as when asleep, but Briicke 
has shown that this movement results from a different 
state of turgescence in the cells from that which 
occurs during sleep ; and as sleep-movements are cer- 
tainly due to modified circumnutation, those from a 
touch can hardly be thus due. The back of a leaf of 
Drosera rotundifolia was cemented to the summit of 
a stick driven into the ground, so that it could not 
move in the least, and a tentacle was observed during 
many hours under the microscope; but it exhibited 
no circumnutating movement, yet after being mo- 
mentarily touched with a bit of raw meat, its basal 
part began to curve in 23 seconds. This curving 
movement therefore could not have resulted from 
modified circumnutation. But when a small object, 
such as a fragment of a bristle, was placed on one side 
of the tip of a radicle, which we know is continually 
circumnutating, the induced curvature was so similar 
to the movement caused by geotropism, that we can 
hardly doubt that it is due to modified circumnu- 
tation. A flower of a Mahonia was cemented to a 
stick, and the stamens exhibited no signs of circum- 
nutation under the microscope, yet when they were 
lightly touched they suddenly moved towards the pistil. 
Lastly, the curling of the extremity of a tendril when 



* See the remarks in Frank's 91, &c), on natural selection in 
; Die wagerechte Kichtung vcn connection with geotropism, helio- 
Pnanzentheilen' '1870, pp. 90, tropism, &o. 



Chap. XII. CONCLUDING KEMARKS. 571 

touched seems to be independent of its revolving 01 
circumnutating movement. This is best shown by th« 
part which is the most sensitive to contact, circum- 
nutating much less than the lower parts, or apparently 
not at all.* 

Although in these cases we have no reason to 
believe that the movement depends on modified cir- 
cumnutation, as with the several classes of movement 
described in this volume, yet the difference between 
the two sets of cases may not be so great as it at 
first appears. In the one set, an irritant causes an 
increase or diminution in the turgescenee of the cells, 
which are already in a state of change ; whilst in the 
other set, the irritant first starts a similar change in 
their state of turgescenee. Why a touch, slight 
pressure or any other irritant, such as electricity, heat, 
or the absorption of animal matter, should modify the 
turgescenee of the affected cells in such a manner as to 
cause movement, we do not know. But a touch acts in 
this manner so often, and on such widely distinct plants, 
that the tendency seems to be a very general one ; and 
if beneficial, it might be increased to any extent. In 
other cases, a touch produces a very different effect, 
as with Nitella, in which the protoplasm may be seen 
to recede from the walls of the cell; in Lactuca, in 
which a milky fluid exudes; and in the tendrils of 
certain Vitaceae, Cucurbitacese, and Bignoniaceae, in 
which slight pressure causes a cellular outgrowth. 

Finally, it is impossible not to be struck with the 
resemblance between the foregoing movements of 
plants and many of the actions performed uncon- 
sciously by the lower animals.f With plants an 

* For the evidence on this pp. 173, 174. 
head, see the ' Movements and f Sachs remarks to nearly the 

Habits of Climbing Plants,' 1875, same effect: " Dass sich die le« 



572 SUMMAEY AND Chap. XII 

astonishingly small stimulus suffices; and even with 
allied plants one may be highly sensitive to the 
slightest continued pressure, and another highly sensi- 
tive to a slight momentary touch. The habit of moving 
at certain periods is inherited both by plants and 
animals ; and several other points of similitude have 
been specified. But the most striking resemblance is 
the localisation of their sensitiveness, and the transmis- 
sion of an influence from the excited part to another 
which consequently moves. Yet plants do not of course 
possess nerves or a central nervous system; and we 
may infer that with animals such structures serve only 
for the more perfect transmission of impressions, and 
for the more complete intercommunication of the 
several parts. 

We believe that there is no structure in plants more 
wonderful, as far as its functions are concerned, than 
the tip of the radicle. If the tip be lightly pressed 
or burnt or cut, it transmits an influence to the upper 
adjoining part, causing it to bend away from the 
affected side ; and, what is more surprising, the tip 
can distinguish between a slightly harder and softer 
object, by which it is simultaneously pressed on oppo- 
site sides. If, however, the radicle is pressed by a 
similar object a little above the tip, the pressed part 
does not transmit any influence to the more distant 
parts, but bends abruptly towards the object. If the 
tip perceives the air to be moister on one side than 
on the other, it likewise transmits an influence to the 
upper adjoining part, which bends towards the source 
of moisture. When the tip is excited by light (though 



bende Pflanzensubstanz derart lich,wiedie verschiedenen Sinnes- 

innerlich differenzirt, dass ein- nerven des Thiere ' (' Arbeit en 

zelne Theile mit specifischen des Bot. Inst, in Wurzlmrg,' Bd, 

Energien ausgeriistot sind, ahn- ii. 1879, p. 282). 



Chap. XII. CONCLUDING REMARKS. 573 

in the case of radicles this was ascertained in only a 
single instance) the adjoining part bends from the 
light ; bnt when excited by gravitation the same part 
bends towards the centre of gravity. In almost every 
case we can clearly perceive the final purpose or advan- 
tage of the several movements. Two, or perhaps more, 
of the exciting causes often act simultaneously on the 
tip, and one conquers the other, no doubt in accord- 
ance with its importance for the life of the plant. 
The course pursued by the radicle in penetrating the 
ground must be determined by the tip; hence it 
has acquired such diverse kinds of sensitiveness. It 
is hardly an exaggeration to say that the tip of the 
radicle thus endowed, and having the power of 
directing the movements of the adjoining parts, acts 
like the brain of one of the lower animals ; the brain 
being seated within the anterior end of the body, 
receiving impressions from the sense-organs, and 
directing the several movements. 



INDEX. 



ABIES. 



Abies communis, effect of killing or 
injuring the leading shoot. 187 

■ pectinata, effect of killing or 

injuring the leading shoot, 187 

, affected by Mcidium elatinum, 

188 

Abronia umbellata, its single, deve- 
loped cotyledon, 78 

, rudimentary cotyledon, 95 

, rupture of the seed coats, 105 

Abutilon Darwinii, sleep of leaves 
and not of cotyledons, 314 

, nocturnal movement of leaves, 

323 

Acacia Farnesiana, state of plant 
when awake and asleep, 381, 382 

, appearance at night, 395 

, nyctitropic movements of 

pinnse, 402 

, the axes of the ellipses, 404 

lophantha, character of first 

leaf, 415 

retinoides, circnmnutation of 

young phyllode, 236 

Acanthosicyos horricla, nocturnal 
movement of cotyledon 304 

Acanthus candelabrum, inequality in 
the two first leaves, 79 

, petioles not arched, 553 

latifolius, variability in first 

leaves. 79 

«— — - mollis, seedling, manner of 
breaking through the ground, 
78, 79 

, circumnutation of young leaf, 

249, 269 

8pinosus, 79 

, movement of leaves, 249 



AMPHICARHBA. 

Adenanthera pavonia, nyctitropie 
movements of leaflets, 374 

JEcidium elatinum, effect on the 
lateral blanches of the silver fir, 
188 

Msculus hfppocastanum, movements 
of radicle, 28, 29 

, sensitiveness of apex of radicle, 

172-174 

Albizzia lophantha, nyctitropic move- 
ments of leaflets, 383 

, of pinnae, 402 

Allium cepa, conical protuberance 
on arched cotyledon, 59 

, circumnutation of basal half 

of arched cotyledon, 60 

, mode of breaking through 

ground, 87 

, straightening process, 101 

porrum, movements of flower- 
stems, 226 

Alopecurus pratensis, joints affected 
by apogeotropism, 503 

Aloysia citriodora, circumnutation 
of stem, 210 

Amaranthus, sleep of leaves 387 

caudatus, noctural movement 

of cotyledons, 307 

Amorpha fruticosa, sleep of leaflets, 
354 

Ampelopsis tricuspidata, hyponastio 
movement of hooked tips, 272- 
275 

Amphicarpoza monoica, circumnuta- 
tion and nyctitropic movements 
of leaves, 365 

, effect of sunshine on leaflets^ 

445 

, geotropic movements of, 

520 



INDEX. 



f>75 



Anoda Wrightii, sleep of cotyledons, 
302,312 

, of leaves, 324 

• , downward movement of coty- 
ledons, 444 

Apheliotropism, or negative helio- 
tropism, 5, 419, 432 

Apios graveolens, heliotropic move- 
ments of hypocoty], 422-424 

tuberosa, vertical sinking of 

leaflets at night, 368 

Apium graveolens, sleep of cotyle- 
dons, 305 

, petroselinum, sleep of cotyle- 
dons, 304 

Apogeotropic movements effected by 
joints or pulvini, 502 

Apogeotropism, 5, 494 ; retarded by 
heliotropism, 501 ; concluding re- 
marks on, 507 

Arachis hypogoea, circumnutation of 
gynophore, 225 

■ , effects of radiation on leaves, 

289, 29o 

, movements of leaves, 357 

, rate of movement, 404 

, circumnutation of vertically 

dependent young gynophores, 519 

■ , downward movement of the 

same, 519 

Arching of various organs, impor- 
tance of, to seedling plants, 87, 
88 ; emergence of hypocotyls or 
epicotyls in the form of an, 553 

Asparagus officinalis, circumnuta- 
tion of plumules, 60-02. 

, effect of lateral light, 484 

Asplenium tridiomanes, movement 
in the fruiting fronds, 257, n. 

Astragalus uliginosus, movement of 
leaflets, 355 

Avena sativa, movement of cotyle- 
dons, 65, 66. 

, sensitiveness of tip of radicle 

to moist air, 183 

— -, heliotropic movement and cir- 
cumnutation of cotyledon, 421,422 

, sensitiveness of cotyledon to a 

lateral light, 477 

, young sheath-like cotyledons 

strongly apogeotropic, 499 



Avena sativa, movements of oldigb 

cotyledons, 499, 500 
Averrhoa bilimbi, leaf asleep, 330 
, angular movements when 

going to sleep, 331-3.15 
■ , leaflets exposed to bright 

sunshine, 447 
Azalea Indica, circumnutation of 

stem, 208 



B. 



Bary, de, on the effect of the Mci- 
dium on the silver fir, 188 

Batiilin, Piof, on the nyctitropic 
movements of leaves, 2^3 ; on the 
sleep of leaves of Sida napoea, 
322 ; on Polygonum aciculare, 
387 ; on the effect of sunshine on 
leaflets of Oxalis aretosella, 447 

Bauhinia, nyctitiopic movements, 
373 

, movementsof petioles of young 

seedlings, 401 

, appearance of young plants 

at night, 402 

Beta vulgaris, circumnutation of 
hypocotyl of seedlings, 52 

, movements of cotyledons, 52, 

53 

, effect of light, 124 

, nocturnal movement of coty- 
ledons, 307 

, heliotropic movements of, 

420 

■ , transmitted effect of light on 

hypocotyl, 482 

. apogeotiopic movement of 

hypocotyl, 496 

Bignonia capreolata, apheliotropic 
movement of tendrils, 432, 45() 

Bouche on Melaleuca ericce.folia 
383 

Brassica napus, circumnutation ol 
flower-stems, 226 

Brassica oleracea, circumnutation: 
of seedling, 10 

, of radicle, 11 

, geotropic movement of radicle. 

11 



576 



INDEX. 



BRASSICA. 

Brassica oleracea, movement of 

buried and arched hypocotyl, 13, 

14, 15 
, conjoint circumnutation of 

hypocotyl and cotyledons, 16, 17, 

18 

, of hypocotyl in darkness, 19 

, of a cotyledon with hypocotyl 

secured to a stick, 19, 20 

, rate of movement, 20 

, ellipses described by hypo- 

cotyls when erect, 105 

, movements of cotyledons, 115 

, of stem, 202 

, of leaves at night, 229, 

230 

, sleep of cotyledons, 30 1 

, circumnutation of hypocotyl 

of seedling plant, 425 
, heliotropic movement and 

circumnutation of hypocotyls, 

426 
, effect of lateral light on hypo- 
cotyls, 479-482 
, apogeotropic movement of 

hypocotyls, 500, 501 
Brassica rapa, movements of leaves, 

230 
Brongniart, A., on the si ep of 

Strephium floribundum, 391 
Bruce, Dr., on the Jeep of leaves in 

Arerrhoa, 330 
Bryophyllum (vel Calanchoe) calyci- 

num, movement of leaves, 237 



C. 



Camellia Japonica, circumnutation 
of leaf, 231, 232 

Candolle, A. de, on Trapa natans, 
95 ; on sensitiveness of coty- 
ledons, 127 

Canna Warscewiczii, circumnuta- 
tion of plumules, 58, 59 

. of leaf, 252 

Cannabis sativa, movements of 
leaves, 250 

, nocturnal movements of coty- 
ledons, 307 



CASSIA. 

Cannabis sativa, sinking of the young 
leaves at night, 444 

Oassii, nyctitropic movement cf 
leaves, 369 

Cassia Barclayana, nocturnal move- 
ment of leaves, 372 

, alight movementof leaflets,40 L 

calliantha, uninjured by ex- 
posure at night, 289. n. 

, nyctitropic movement of 

leaves, 371 
—. — , circumnutating movement of 

leaves, 372 

corymbosa, cotyledon-3 sensi- 
tive to contact, 126 

, nyctitropic movement of 

Laves, 369 

floribunda, use of sleep move- 
ments, 289 

, effect of radiation on the 

leaves at night, 294 

, circumnutating and nycti- 
tropic movement of a terminal 
leaflet, 372, 373 

, movements of young and older 

leaves, 400 

florida, cotyledon's sensitive to 

contact, 126 

, sleep of cotyledons, 308 

glauca, cotyledons sensitive to 

contact, 126 

, sleep of cotyledons, 308 

laevigata, effect of radiation 

on leaves, 289, n. 

rnimosoides, movement of coty- 
ledons 116 

, sensitiveness of, 126 

, sleep of, 308 

, nyctitropic movement of 

leaves, 372 

, effect of bright sunshine on 

cotyledons, 446 

neglecta, movements of, 117 

, effect of lig! it, 124 

, sensitiveness of cotyledons 

126 

nodosa, non-sensitive cotyle- 
dons, 126 

, do not rise at nisrht, 308 

pube^eens, non-sensitive coty^ 

ledons, 126 



INDEX 



577 



CASSIA. 

Cassia pubescens, uninjured by ex- 
posure at night, 293 

, sleep of cotyledons, 308 

, nyctitropic movement of 

Jeaves, 371 

, circum nutating movement 

of leaves, 372 

, nyctitropic movement of 

petioles, 400 

■ , diameter of plant at night, 

402 

sp. (?) movement of cot vledons, 

116 

tora, circnmnutation of coty- 
ledons and hypocotyls, 34, 35, 
109, 308 

. , effect of light, 124, 125 

, sensitiveness to contact, 

125 

, heliotropic movement and 

circumnutation of hypocotyl, 
431 

, hypocotyl of seedling slightly 

heliotropic, 454 

, apogeotropic movement of old 

hypocotyl, 497 

, movement of hypocotyl of 

young seedling, 510 

Caustic (nitrate of silver), effect of, 
on radicle of bean, 150, 156 ; on 
the common pea, 160. 

Cells, table of the measurement 
of, in the pulvini of Oxalis 
corniculata, 120 ; changes in, 
547 

Centrosema. 365 

Ceratophyllum demersum, move- 
ments of stem, 211 

Cereus Landbeckii, its rudimentary 
cotyledons, 97 

speciossimus, circumnutation 

of stem, 206, 207 

Cerinthe major, circumnutation of 
hypocotyl, 49 

, of cotyledons, 49 

, ellipses described by hypo- 
cotyls when erect, 107 

■ effect of darkness, 124 

Chatin, M., on Pinus Nordmon- 
niana, 389 

Chenopodium album, sleep of 



ORINUM. 

leaves, but not of cotyledons, 314. 
319 

Chenopodium album, movement of 
leaves, 387 

Chlorophyll injured by bright light, 
44(3 

Ciesielski, on the sensitiveness of 
the tip of the radicles, 4, 523 

Circumnutation, meaning explained 
1 ; modified, 263-279 ; and helio- 
tropism, relation between, 435 ; 
of paramount importance to every 
plant, 547 

Cissus discolor, circumnutation of 
leaf, 233 

Citrus aurantium, circumnutation 
of epicotyl, 28 

, unequal cotyledons, 95 

Clianihus Dumpieri, nocturnal 
movement of leaves, 297 

Coboza scandens, circumnutation of, 
270 

Cohn, on the water secreted by 
Lathreea squamaria, 86, n. ; on 
the movement of leaflets of Oxa- 
lis, 447 

Colutea arborea, nocturnal move- 
ment of leaflets, 355 

Conifer x, circumnutation of, 211 

Coronilla rosea, leaflets asleep, 355 

Corylus avellana, circumnutation of 
young shoot, emitted from the 
epicotyl, 55, 56 

, arched epicotyl, 77 

Cotyledon umbilicus, circumnuta- 
tion of stolons, 219, 220 

Cotyledons, rudimentary, 94-98 ; 
circumnutation of, 109-112; noc- 
turnal movements, 111, 112 ; pul- 
vini or joints of, 112-122; dis- 
turbed periodic movements by 
light, 123 ; sensitiveness of, to 
contact, 125 ; nvctitropic move- 
ments of, 283, 297 ; list of coty- 
ledons which rise or sink at 
night, 300 ; concluding remarks 
on their movements, 311 

Crambe maritima, circumnutation of 
leaves, 228, 22y 

Crinnm cape use, shape (if leaves, 
253 



hip 



INDEX. 



CEDttJM. 

Crinum capense, circumnutation of, 
254 

Crutolaria (sp. ?), sleep of leaves, 
340 

Cryptogams, circunmutation of, 
257-259 

Cucumis dudaim, movement of coty- 
ledons, 43, 44 

, sleep of cotyledons, 304 

Cucurbita aurantia, movement of 
hypocotyl, 42 

, cotyledons vertical at night, 

304 

ovifera, geo tropic movement 

of radicle, 38, 39 

, circunmutation of arched hypo- 
cotyl, 39 

, of straight and vertical hypo- 
cotyl, 40 

, movements of cotyledons, 41, 

42, 115, 124 

, position of radicle, 89 

, rupture of the seed - coats, 

102 

, circunmutation of hypocotyl 

when erect, 107, 108 

, sensitiveness of apex of radi- 
cle, 169-171 

, cotyledons vertical at night, 

304 

, not affected by apogeotropism, 

509 

, tips cauterised transversely, 

537 

Curvature of the radicle, 193 

Cyeas pectinata, circunmutation of 
young leaf, whilst emerging from 
the ground, 58 

, first leaf arched, 78 

, circumnutatiou of terminal 

leaflets, 252 

Cyclamen Persicum, movement of 
cotyledon, 46 

, undeveloped cotyledons, 78, 

96 

, circunmutation of peduncle, 

225 

, , of leaf, 246, 247 

, dowuward apheliotropic move- 
ment of a flower peduncle, 433- 
435 



DESMODIUM. 

Cyclamen Persicum, burying of the 

pods, 433 
Cyperus alternifolius, circunmuta- 
tion of stem, 212 

, movement of stem, 509 

Cytisus fragrans, circumnutation of 
hypocotyl, 37 

, sleep of leaves, 344, 397 

, apogeotropic movement of 

stem, 491-496 



of young 
de- 



D. 



Dahlia, circumnutation 
leaves, 244-246 

Dalea alopecuroides, leaflets 
pressed at night, 354 

Darkness, effect of, on the move- 
ment of leaves, 407 

Darlingtonia Calif ornica, its leaves 
or pitchers apheliotropic, 450, n. 

Darwin, Charles, on Maurandia 
semper ft >rens, 225; on the Swedish 
turnip, 230, n. ; movements of 
climbing plants, 266. 271; the 
heliotropic movement of the ten- 
drils of Bignonia capreolata, 433 ; 
revolution of climbing plants, 
451 ; on the curling of a tendril, 
570 

, Erasmus, on the peduncles of 

Cyclamens, 433 

, Francis, on the radicle of 

Sinapis alba, 486 ; on Hygrosco- 
pic seeds, 489, n. 

Datura stramonium. nocturnal 
movement of cotyledons, 298 

Delpino, on cotyledons of Chaero- 
phyllum and Corydalis, 96, n. 

Delphinium nudicaule, mode of 
breaking through the ground, 80 

, confluent petioles of two coty- 
ledons, 553 

Desmodium gyrans, movement of 
leaflets, 257, n. 

, position of leaves at night, 

285 

, sleep of leaves, not of coty 

ledons, 314 

, circumnutation and nycti- 



INDEX 



579 



UE8MODIU2I. 

tropic movement of leaves, 358- 
860 

Desmodium gyrans, movement of 
lateral leaflets, 361 

, jerking of leaflets, 362 

, nyctitropic movement of peti- 
oles, 400, 401 

— — , diameter of plant at night, 
402 

, lateral movement of leaves, 

404 

, zigzag movement of apex of 

leaf, 405 

■ , shape of lateral leaflet, 416 

vespertilionis, 364, n. 

Deutzia gracilis, circumnutation of 
stem, 205 

Diageotropism, 5 ; or transverse- 
geotropism, 520 

DialiL-liotropism, 5; or Transversal- 
Heliotropismus of Frank, 419 ; 
influenced by epinasty, 439 ; 
by weight and apogeotropism, 
440 

Dianthus caryophyllus, 230 

, circumnutation of young leaf, 

231, 269 

Dicotyledons, circumnutation wide- 
ly spread among, 68 

Dioncea, oscillatory movements of 
leaves, 261, 271 

Dioncea rnuscipula, circumnutation 
of young expanding leaf, 239, 
240 

, closure of the lobes and cir- 
cumnutation of a full-grown leaf, 
241 

, oscillations of, 242-244 

Diurnal sleep, 419 

Drosera Capensis, structure of first- 
formed leaves, 414 

rotundifolia, movement of 

young leaf, 237, 238 

, of the tentacles, 239 

, sensitiveness of tentacles, 

261 

, shape of leaves, 414 

, leaves not heliotropic, 450 

, leaves circumnutate largely, 

454 
, sensitiveness of 570 



EUCALYPTUS. 

Duchartre on Tephrosia caribma, 
354; on the nyctitropic nioveini nt 
of the Cassia, 369 

Duval-Jouve, on the movements of 
Bryophyllum calycinum, 237 ; of 
the narrow leaves of the Grami- 
nese, 413 

Dyer, Mr. Thiselton, on the leaves 
of Crotolaria, 340 ; on Cassia flori- 
bunda, 369, n.. on the absorbent 
hairs on the buried flower-heads 
of Trifolium subterraneum, 517 



E 



Echeveria stolonifera, circumnuta- 
tion of leaf, 237 

Echinocactus viridescens, its rudi- 
mentary cotyledons, 97 

Echinocystis lobata, movements of 
tendrils, 266 

, apogeotropism of tendrils, 

510 

Elfving, F., on the rhizomes of 
Sparganium ramosum, 189; on 
the diageotropic movement in the 
rhizomes of some plants, 521 

Elymus arenareus, leaves closed 
during the day, 413 

Embryology of leaves, 414 

Engelmann, Dr., on the Quercus 
virens, 85 

Epinasty, 5, 267 

Epicotyl. or plumule, 5 ; manner 
of breaking through the ground, 
77 ; emerges from the ground 
under the form of an arch, 553 

Erythrina caffra, sleep of leaves, 
367 

corallodendron, movement of 

terminal leaflet, 367 

crista-galli, effect of tem« 

perature on sleep of leaves, 
318 

, circumnutation and nycti- 
tropic movement of terminal 
leaflets, 367 

Eucalyptus resinifera, circumnuta* 
tion of loaves, 244 



580 



INDEX. 



EUPHORBIA. 



Euphorbia jacquinexflara, nycti- 
tropic movement of leaves, 388 



Flahault, M., on the rupture of 
seed-coats, 102-104, 106 

Flower- stems, circumnutation of, 
2-23-226 

Fragaria Rosacea, circumnutation 
of stolon, 214-218 

Frank, Dr. A. B., (he terms Helio- 
tropism and Geotropism, first 
used by him, 5, n. ; radicles acted 
on by geotropism, 70, n. ; on the 
stolons of Fragaria, 215; periodic 
and nyctitropic movements of 
leaves, 284; on the root-leaves 
of plants kept in darkness, 443 ; 
on pulvini, 485 ; on natural 
selection in connection with 
geotropism, heliotropism, &c, 
570 

, on Transversal- Heliotropis- 

mus, 419 

Fuchsia, circumnutation of stem, 
205, 206 



G. 



circumnutation 



Gazania ringens, 

of stem, 208 
Genera containing sleeping plants, 

320, 321 
Geotropism, 5 ; effect of, on the 

primary radicle, 196 ; the reverse 

of apngeotropism, 512 ; effect on 

the tips of radicles, 543 
Geranium cinereum, 304 

Endressii, 304 

Ibericum, nocturnal movement 

of cotyledons, 298 

■ Richardsoni, 304 

■ rotundifolium, nocturnal move- 
ment of cotyledon, 304, 312 

subcaulescens, 304 

Germiuating seed, history of a, 
548 



GYMNOSPERM8. 

Githago segetum, circumnutation of 
hypocotyl, 21, 108 

, burying of hypocotyl, 109 

, seedlings feebly illuminated, 

124, 128 

, sleep of cotyledon, 302 

, leaves, 321 

Glaucium luteum, circumnutation 
of young leaves, 228 

Gleditschia, sleep of leaves, 3G8 

Glycine hispida, vertical sinking of 

• leaflets, 366 

Glycyrrhiza, leaflets depressed at 
night, 355 

Godlewskl, Emil, on ,the tume- 
scence of the cells, 485 

Gooseberry, effect of radiation, 281 

Gossypium (var. Nankin cotton), 
circumnutation of hypocotyl, 
22 

, movement of cotyledon, 22, 23 

, sleep of leaves, 324 

arboreum (?), sleep of cotyle- 
dons, 303 

Braziliense, nocturnal move- 
ment of leaves, 324 

, sleep of cotyledons, 303 

herbaceum, sensitiveness of 

apex of radicle, 168 

, radicles cauterised trans- 
versely, 537 

maritimum, nocturnal move- 
ment of leaves, 324 

Gravitation, movements excited by, 
567 

Gray, Asa, on Delphinium nudi- 
caule, 80; on Megarrhiza Cali- 
fornica, 81 ; on the movements in 
the fruiting fronls of Asplenium 
trichomanes, 257 ; on the Amphi- 
carpcea monoica, 520 ; on the 
Ipomoza Jalappa, 557 

Grease, effect of, on radicles and 
their tips, 182, 185 

Gressner, Dr. H., on the cotyledons 
of Cyclamen Persicum, 46, 77 * 
on hypocotyl of the same, 96 

Gymnosperms, 389 



INDEX. 



581 



HABEBLANDT. 



H. 



Haberlandt, Dr., on the protube- 
rance on the hypocotyl of Allium, 
59 ; the importance of the arch 
to seedling plants, 87 ; sub- 
aerial and subterranean cotyle- 
dons, 110, n. ; the arched hypo- 
cotyl, 554 

Hxmatoxylon Campechianum. noc- 
turnal movement of leaves, 368, 
369 

Hedtra helix, circumnutatiou of 
stem, 207 

Hedysarum coronarium, nocturnal 
movements of leaves, 356 

Eelianthemum prostratum, geotro- 
pic movement of flower-heads, 
518 

Helianthus annuus, circumnutatiou 
of hypocotyl, 45 

, arching of hypocotyl, 90 

■ , nocturnal movement of coty- 
ledons, 305 

Heliotropism, 5 ; uses of, 449 ; a 
modified form of circumuutation, 
490 

Helleborus 7iiger t mode of breaking 
through the ground, 86 

Hensen, Prof., on roots in worm- 
burrows, 72 

Henslow, Rev. G., on the coty- 
ledons of PhalarU Canariensis, 
62 

Hofmeister, on the curious move- 
ment of Spirogyra, 3, 259, n. ; of 
the leaves of Pistia stratiotes, 
255 ; of cotyledons at night, 297 ; 
of petals, 414 

— — and Batalin on the movements 
of the cabbage, 229 

Hooker, Sir J., on the effect of light 
on the pitchers of Sarracenia, 
450 

Hypocotyl, 5 ; manner of break- 
ing through the ground, 77 ; 
emerges under the form of an 
arch, 553 

Hypocotyls and Epicotyls, circum- 



IPOMlBA. 

nutation and other movements 
when arched.98; power of straight- 
ening themselves, 100; rupture 
of the seed-coats, 102-10G ; illus- 
tration of, 106 ; circumnutatiou 
when erect, 107 ; when in dark 
108 
Hyponasty, 6, 267 



I. 



Iberis umbellata, movement of stem, 
202. 

Illumination, effect of, on the sleep 
of leaves, 398 

Imatopliyllum vel Clivia (sp. ?), 
movement of leaves, 255 

Indigofera tinctoria, leaflets do- 
pressed at night, 351 

Inheritance in plants, 407, 491 

Insectivorous and climbing plants 
not heliotropic, 450 ; influence of 
light on, 488 

Ipomcea bona nox, arching of hypo- 
cotyl, 90 

, nocturnal position of coty- 
ledons, 306, 312 

coerulea vel Pharbitis nil, 

circumnutation of seedlings, 
47 

, movement of cotyledons, 47- 

49, 109 

, nocturnal movements of coty- 
ledons, 305 

, sleep of leaves, 386 

, sensitiveness to light, 451 

, the hypocotyledonous stems 

heliotropic, 453 

, coccinea, position of coty- 
ledons at night, 306, 312 

leptophylla, mode of breaking 

through tie ground, 83, 84 

, arching of the petioles of the 

cotyledons, 90 

, difference in sensitiveness to 

gravitation in different parts, 
509 

, extraordinary manner of ger- 
mination, 557 



582 



INDEX. 



IPOMCEA. 

Ipomcea pandurata, manner of ger- 
mination, 84, 557 

purpurea (vel Pharbitis liis- 

pida), nocturnal movement of 
cotyledons, 305, 312 

— , sleep of leaves, 386 

, sensitiveness to light, 451 

, the hypocotyledonous stems 

heliotropic, 453 

Iris pseudo-acoruSy circumnutation 
of leaves, 253 

Irmisch, on cotyledons of Ranun- 
culus Ficaria, 96 

Ivy, its stems heliotropic, 451 



K. 



Kemer on the bending down of pe- 
duncles, 414 

Rlinostat, the, an instrument de- 
vised by Sachs to eliminate geo- 
tropism, 93 

Kraus, Dr. Carl, on the underground 
shoots of Triticum repens, 189; 
on Cannabis sativa, 250, 307, 
312 ; on the movements of leaves, 
318 



L. 



Lactuca scariola, sleep of cotyle- 
dons, 305 

Lagenaria vulgaris, circumnutation 
of seedlings, 42 

, of cotyledons, 43 

, cotyledons vertical at night, 

304 

Lathrsea squamaria, mode of 
breaking through the ground, 
85 

, quantity of water secreted, 

85, 86, to. 

T.uthyrus nissolia, circumnuta- 
tion of stem of young seedling, 
33 

, ellipses described by, 107, 

108 

Leaves, circumnutation of, 226- 



262 ; dicotyledons, 226-252 ; mo 
nocotyledons, 252-257 ; nyctitro- 
pism of, 280 ; their temperature af- 
fected by their position at night, 
294 ; nyctitropic or sleep move- 
ments, 315, 394 ; periodicity of 
their movements inherited, 407; 
embryology of, 414; so-called 
diurnal sleep, 445 

Leguminosx, sleep of cotyledons, 
308 ; sleeping speeies, 340 

Le Maout and Decaisne, 67 

Lepidium sativum, sleep of cotyle- 
dons, 302 

Light, movements excited by 418, 
563 ; influence on mo.it vegetable 
tissues, 486 ; acts on plant as on 
the nervous system of animals, 
487 

Lilium auratum, circumnutation of 
stem, 212 

, apogeotropic movement of 

stem, 498, 499 

Linnaeus, 'Somniis Plantarum,' 
280 ; on plants sleeping, 320 ; 
on the leaves of Slda abutilon, 
324; on Oenothera mollissima, 
383 

Linum Berendieri, nocturnal move- 
ment of cotyledons, 298 

usitatissimum, circumnutation 

of stem, 203 

Lolium perenne, joints affected by 
apogeotropism, 502 

Lonicera brachypoda, hooking of the 
tip, 272 

, sensitiveness to light. 453 

Loomis, Mr., on the movements in 
the fruiting fronds of Asplenium 
trichomanes, 257 

Lotus aristata, effect of radiatkn 
on leaves, 292 

Creticus, leaves awake and 

asleep, 354 

Gebelii, nocturnal movement 

of cotyledons, 308 

, leaflets provided with pulvini, 

353 
Jacobzeus, movements of coty 

ledons, 35, 109 
, pulvini of, 115 



INDEX. 



583 



LOTUS. 


MELILOTUS. 


Lotus Jacobseus, movements at 


Marsilia quadrifoliata, effect of ra- 


night, 116, 121. 124 


diation at night, 292 


, development of pulvini, 122 


, circumnutation and nycti- 


, sleep of cotyledons, 308, 313 


tropic movement of leaflets, 3J2- 


, nyctitropic movement of 


394 


leaves, 353 


, rate of movement, 404 


major,, sleep of leaves, 353 


Martins, on radiation at night, 


perigrinus, movement of leaf- 


284, n. 


lets, 353 


Masters, Dr. Maxwell, on the lead- 


Lunnlaria vulgar «, circumnutation 


ing shoots of the Coniferx, 211 


of fronds, 258 


Maurandia semperflorens, circumnu* 


Lupinus, 340 


tation of peduncle, 225 


albifrons, sleep of leaves, 344 


Medicago maculata, nocturnal posi- 


Hartwegii, sleep of leaves, 


tion of leaves, 345 


341 


marina, leaves awake and 


lateus, circumnutation of coty- 


asleep, 344 


ledons, 38, 110 


Meehan, Mr., on the effect of an 


, effect of darkness, 124 


iEcidium on Portulaca oleracea, 


Lupinus, posiiion of leaves when 


189 


asleep, 341 


Megarrhiza Californica, mode of 


, different positions of leaves at 


breaking through the ground, 


night, 343 


81 


, varied movements of leaves 


, germination described by Asa 


and leaflets, 395 


Gray, 82 


Menziesii, sleep of leaves, 343 


, singular manner of germina- 


mutabilis, sleep of leaves, 


tion, 83, 556 


343 


Melaleuca ertcoefolia, sleep of leaves, 


nanus, sleep of leaves, 343 


383 


pilosus, sleep of leaves, 340, 


Melilutus, sleep of leaves, 345 


341 


alba, sleep of leaves, 347 


polyphyllus, sleep of leaves, 


cazrulea, sleep of leaves, 347 


343 


dentata, effect of radiation at 


pubescens, sleep of leaves by 


night, 295 


day ami night, 342 


elegans, sleep of leaves, 347 


, position of petioles at night, 


gracilis, sleep of leaves, 347 


343 


infesta, sleep of leaves, 347 


* ■ , movements of petioles, .401 


Italica, leaves exposed at 


speciosus, circumnutation of 


night, 291 


leaves, 236 


-, sleep of leaves, 347 


Lynch, Mr. E., on Puchira aqua- 


macrorrhiza, leaves exposed at 


tica, 95, n. ; sleep movements of 


night, 292 


Averrhoa, 330 


, sh ep of leaves, 347 




messanensis, sleep of leaves on 




full-grown and young plants, 


M. 


348, 416 


officinalis, effect of exposure of 




leaves at night, 290, 296 


fflaranta arundinacea, nyctitropic 


, nocturnal movement of leaves. 


movement of leaves, 389-391 


346, 347 


, after much agitation do not 


, circumnutation of leaves, 348 


sleep, 319 


, movement of petioles, 401 



38 



584 



INDEX. 



MELILOTUS. 

Melilotus parviflora, sleep of .eaves, 
347 

Petitpierreana, leaves exposed 

at night, 291, 296 

, sleep of leaves, 347 

secundiflora, sleep of leaves, 

347 

suaveolens, leaves exposed at 

night, 291 

, sleep of leaves, 347 

sulcata, sleep of leaves, 347 

Taurica, leaves exposed at 

night, 291 

, sleep of leaves, 347, 415 

Methods of observation, 6 

Mimosa albida, cotyledons vertical 

at night, 116 

, not sensitive to contact, 127 

, sleep of cotyledons, 308 

, rudimentary leaflets, 3d4 

, nyctitropic movements of 

leaves, 379, 380 
, circumnutation of the main 

petiole of young leaf, 381 
, torsion, or rotation of leaves 

and leaflets, 400 

, first true leaf, 416 

, effect of br'ght sunshine on 

basal leaflets, 445 

marginata, nyctitropic move- 
ments of leaflets, 381 

pudica, movement of coty- 
ledons, 105 

■ , rupture of the seed-coats, 

105 
, circumnutation of cotyledons, 

109 

, pulvini of, 113, 115 

, cotyledons vertical at night, 

116 
, hardly sensitive to contact, 

127 
, effect of exposure at night, 

293 
, nocturnal movement of leaves, 

297 

, sleep of cotyledons, 308 

, circumnutation and nycti- 
tropic movement of main petiols, 

374-378 
, of leaflets, 378 



NEPTUNIA. 

Mimosa albida, circumnutation and 
nyctitropic movement of piunaa, 
402 

, number of ellipses described 

in given time, 406 

, effect of bright sunshine on 

leaflets, 446 

Mirabilis jalapa and longiflora- 
nocturnal movements of cotyle- 
dons, 307 

, nyctitropic movement of 

. leaves," 387 

Mohl, on heliotropism in ten- 
drils, stems, and twining plants, 
451 

Momentnm-like movement, the ac- 
cumulated effects of apogeo- 
tropi m, 508 

Monocotyledons, sleep of leaves. 
389 

Monotropa hypopitys, mode of 
brc aking through the ground, 8C 

Morren, on the movements of 
stamens of Sparmannia and 
Cereus, 226 

MtilL r, Fritz, on Cassia tora, 34 ; 
on the circumnutation of Linum 
usitatissimum, 203 ; movements 
of the flower-stems of an Alisma, 
226 

Mutisia clematis, movement of 
leaves, 246 

, leaves not heliotropic, 451 



N. 



Natural selection in connection 
with geotropism, heliotropism, 
&c, 570 

Nephrodium molle, circumnutation 
of very young frond, 63 

, of older frond, 257 

, slight movement of fronds 

509 

Neptunia oleracea, sensitiveness to 
conta t, 128 

, nvctitropic movement of leaf- 
lets, 374 

, of pinna}, 402 



INDEX. 



585 



KICOTIANA. 

Nicot/ana glauca, sleep of leaves, 
385, 386 

■ , circumnutation of leaves, 

386 

Nobbe, ou the rupture of the seed- 
coats iu a seedling of Marty nia, 
105 

Nolana prostrata, movement of seed- 
lings in the dark, 50 

, circumnutation of seedling, 
108 

Nyctitropic movement of leaves, 
560 

Nyctitropism, or sleep of leaves, 
281 ; in connection with radia- 
tion, 286; object gained by it, 
413 



0. 



Observati n, methods of, 6 

GEnotheramollissima, sleep of leaves, 
383 

Opuntia basilar/'s, conjoint circum- 
nutation of hypocotyl and coty- 
ledon, 44 

- thickening of the hypocotyl, 
96 

, circumnutation of hypocotyl 

when erect, 107 

, burying of, 109 

Orauge, seedling, circumnutation 
of, 510 

Orchis pyramidalis, complex move- 
ment of pollinia, 489 

Oxalis acetosella, circumnutation of 
flower-stem, 224 

, effect* of exposure to radia- 
tion at night, 287, 288, 296 

, circumnutation and nycti- 

tropj-; movement in full-grown 
leaf, 326 

■ , circumnutation of leaflet when 

asleep, 327 

- — , rate of circumnutation of 
leaflets, 404 

, effect of sunshine on leaflets, 

417 

■ , circumnutation oi peduncle, 

506 



OXALIS. 

Oxalis acetosella, seed-capsules, only 
occasionally buried, 518 

arttculata, nocturnal move- 
ments of cotyledons, 307 

(Biophytum) sensitiva, ra- 
pidity of movement of cotyledon* 
during the day, 26 

, pulvinus of, 113 

, cotyledons vertical at night, 

116, 118 

bupleurifolia, circumnutation 

of foliaceous petiole, 328 

, nyctitropic movement of ter- 
minal leaflet, 329 

carnosa, circumnutation of 

flower-stem, 223 

, epinastic movements of flower- 
stem, 504 

, effect of exposure at night, 

288, 296 

, movements of the flower-pe- 
duncles due to apo^totropism 
and other forces, 503-506 

corniculata (var. cuprea), 

movements of cotyledons, 26 

— — , rising of cotyledons, 116 

, rudimentary pulviiri of coty- 
ledons, 119 

, development of pulvinus, 

122 

, effect of dull light, 121 

, experiments on leaves at night, 

288 

jloribunda, pulvinus of coty- 
ledons, 114 

. nocturnal movement, 118, 

307, 313 . 

fragrans, sleep of leaves, 

324 

Ortegesii, circumnutation of 

flower stems, 224 

, sleep of large leaves, 327 

, diameter of plant at night, 

402 
, large leaflets affected by bright 

sunshine, 447 

Plumierv, sleep of leaves, 327 

purpurea, exposure of leaflets 

at night, 293 
rosea, ciicumnutation of coty 

leduiis, 23, 24 



686 



INDEX. 



0XALI8. 


PHASEOLUS. 


OxaMs rosea, pulvinus of, 113 


ment and circumnutation of very 


, movement of cotyledons at 


voting leaf, 248, 249, 269. 


night. 117, 118. 307 


Pfeffer, Prof., on the turgescence of 


, effect of dull light, 124 


the cells, 2 ; on pulvini of leaves, 


, non - sensitive cotyledons, 


113, 117; sleep movements of 


127 


leaves, 280, 283, 284; nocturnal 


sensitiva, movement of coty- 


rising of leaves of Malva, 324 ; 


ledons, 109, 127, 128 


movements of leaflets in Desmo- 


, circumnutation of flower-stem, 


dium gyrans, 358; on Phyllan- 


224 


thus Niruri, 388; influence of a 


, nocturnal movement of coty- 


pulvinus on leaves, 3y6 ; periodic 


ledons, 307, 312 


movements of sleeping leaves, 


, sleep of leaves, 327 


407, 40S ; movements of petals, 


tropceoloides, movement of co- 


414 ; effect of bright sunshine on 


tyledons at night, 118, lzO 


leaflets of Kobinia, 445 ; effect of 


Yaldiviana, conjoint circum- 


light on parts provided with pul- 


nutation of cotyledons and hypo- 


vini, 363 


cotyl, 25 


Phalaris Canariensis, movements ol 


, cotyledons rising vertically at 


old seedlings, 62 


night, 114,115, 117, 118 


, circumnutation of cotyledons, 


, non-sensitive cotyledons, 127 


63, 64, 108 


, nocturnal movement of coty- 


, heliotropic movement and cir- 


ledon, 307, 312 


cumnutation of cotyledon towards 


, sleep of leaves and not of co- 


a dim lateral light, 427 


tyledons, 315 


, sensitiveness of cotyledon to 


— — , movements of leaves, 327 


light, 455 




, effect of exclusion of light 




from tips of cotyledons, 456 




, manner of bending towards 




light, 457 


P. 


, effects of painting with Indian 




ink, 467 


PacMra aquatica, unequal cotyle- 


, transmitted effects of light, 


dons, 95, n. 


469 


Pancratium littorale, movement of 


, lateral illumination of tip, 


leaves, 255 


470 


Paraheliotropism, or diurnal sleep 


, apogeotropic movement of the 


of leaves, 445 


sheath-like cotyledons, 497 


Passiflora gracilis, circumnutation 


, change from a straight up- 


and nyctitropic movement of 


ward apogeotropic course to cir- 


leaves, 383, 384 


cumnutation, 499 


, apogeotropic movement of 


, apogeotropic movement of 


tendrils, 510 


cotyledons, 500 


, sensitiveness of tendrils, 550 


Phaseolus Hernandesii, nocturnal 


Pelargonium zonale, cii*3umnutation 


movement of leaves and leaflets, 


of stem, 203 


368 


, and downward movement of 


caracalla, 93 


young leaf, 232, 233, 269 


, nocturnal movement of leaves, 


Petioles, the, rising of, beneficial to 


368 


plant at night, 402 


, effect of bright sunshine on 


Petunia violacea, downward move- 


leaflets, 446 



INDEX. 



587 



PHASEOLUS. 


QUEROUS. 


Phaseolus multiflorus, movement of 


Plants, climbing, circumnutation of, 


radicles, 29 


264 ; movements of, 559 


, of young radicle, 72 


, mature, circumnutation of. 


, of hypocotyl, 91, 93 


201-214 


, sensitiveness of apex of radicle, 


Pliny on the sleep-movements of 


163-167 


plants, 28i) 


> , to moist air, 181 


Plumbago Capensis, circumnutation 


, cauterisation and grease on 


of stem, 208, 209 


the tips, 535 


Poinciana Gilliesii, sleep of leaves, 


, nocturnal movement of leaves, 


368 


368 


Polygonum aviculare, leaves vertical 


, nycti tropic movement of the 


at night, 387 


first imifoliite leaves, 397 


convolvulus, sinking of the 


Roxburghii, effect of bright 


leaves at night, 318 


bunshiae on first leaves, 445 


Pontederia (sp. ?), circumnutation 


, vulgaris 93 


of leaves, 256 


, bleep of leaves, 318 


Porlieria hygrometrica, circum- 


, vertical sinking of leaflets at 


nutation and nyctitropic move- 


night, 368 


ments of petiole of leaf, 335, 


Phyllanthus Niruri, sleep of leaf- 


336 


lets, 388 


, effect of watering, 336-338 


• linoides, sleep of leaves, 


, leaflets closed during the day, 


387 


413 


Pilocereus Houlletii, rudimentary' 


Portulaca oleracea, effect of ^Eci- 


cotyledons, 97 


dium on, 189 


Pimelia spectabilis, sleep of leaves, 


Primula Sinensis, conjoint circum- 


387 


nutation of hypocotyl and coty- 


Pincers, wooden, through which 


ledon, 45, 46 


the radicle of a bean was allowed 


Pringsheim on the injury to chloro- 


to grow, 75 


phyll, 446 


Pinus austriaca. circumnutation of 


Prosopis, nyctitropic movements of 


leaves, 251, 252 


leaflets, 374 


> Nordmanniana, nyctitropic 


Psoralea acaulis, nocturnal move- 


movement of leaves, 389 


ments of leaflets, 354 


pinaster, circumnutation of 


Pteris aquilina, rachis of, 86 


hypjcotyl, 56 


Pulvini, or joints ; of cotyledons, 


— — , movement of two opposite 


112-122; influence of, on the 


cotyledons, 57 


movements of cotyledons, 313; 


■ , circumnutation of young leaf, 


effect on nyctitropic movements, 


250, 251 


396 


, epinastic downward move- 




ment of young leaf, 270 




Fisiia stratiotes, movement of 


Q. 


loaves, 255 


risum sativum, sensitiveness of 




apex of radicle, 158 


Quercus (Ameiican sp.), circumnu* 


, tips of radicles cauterised 


tation of young stem, 53, 54 


transversely, 534 


robur, movement of radicles, 


Plants, sensitiveness to light, 


54, 55 


449; hygroscopic movements of, 


, sensitiveness of apex of 


489 


radicle, 174-176 



588 



INDEX. 



QUERCUS. 

Quercus virens, manner of germina- 
tion, 85, 557 



Radiation at night, effect of, on 
leaves, 284-286 

Radicles, manner in which they 
penetrate the ground. 69-77 ; cir- 
cumnutation of, 69 ; experiments 
with split sticks, 74 : with 
wooden pincers, 75; sensitiveness 
of apex to contact and other irri- 
tants, 129 ; of Vicia faba, 132- 
15S; various experiments, 135- 
140; summary of results, 143-151; 
power of an irritant on, com- 
pared with geotropism, 151-154; 
sensitiveness of tip to moist 
air, 180 ; with greased tips, 
185 ; effect of killing or injuring 
the primary radicle, 187-191 ; 
curvature of, 193; affected by 
moisture, 108 ; tip alone sensitive 
to geotropism, 540 ; protrusion 
and circumnutation in a g< rmina- 
ting seed, 548; tip highly sen- 
sitive, 550 ; the tip acts like the 
brain of one of the lower animals, 
573 

, secondary, sensitiveness of 

the tips in the bean, 151; become 
vertically geotropic, 186-191 

Ramey on the movements of the 
cotyledons of Mimosa pudica, 
and Clianthus DampieH at night, 
297 

Banunculus Ficaria, mode of 
breaking through the gruund, 
86,90 

, single cotyledon, 96 

, effect of lateral light, 484 

Itaplianus sativa, s nsitiveness of 
apex of radicle, 171 

, sleep of cotyledons, 301 

Rattan, Mr., on the germination of 
the seeds of Megarrhiza Califor- 
nica, 82 

Relation between circumnutation 
and heliotropism, 435 



Beseda odorata, hypocotyl of seed- 
ling slightly heiiotropic, 454 

Reversion, due to mutilation, 1 90 

Bhipscdis cassytha, rudimentary co* 
tyledons, 97 

Bicinus Borboniensis, circumnuta- 
tion of arched hypocotyl, 53 

Bobinia, effect of bright sunshine 
on its leaves, 445 

pseudo-acacia, leaflets vertical 

at night, 355 

Rodier, M., on the movements of 
Ceratophyllum demersum, 211 

Royer, Oil., on the sleep-movements 
of plants, 281, n. ; on the sleep of 
h aves, 318 : the leaves of Medi- 
cago maculata, 345 ; on Wistaria 
Sinensis, 354 

Bubus idceus (hybrid) circumnuta- 
tion of stem', 205 

, apogeotropic movement of 

stem, 498 

Ruiz and Pavon, on Porlieria hy- 
grometrica, 336 



S. 



Sachs on " revolving nutation," 1 ; 
intimate connection between tur- 
gescence and growth, 2, n. ; coty- 
ledon of the onion, 59 ; adapta- 
tion of root-hairs, 69 ; the move- 
ment of the rad.cle, 70, 72, 73; 
movement in the hypocotyls of 
the bean, &o, 91 ; sensitiveness 
of radicles, 131, 145, 198; sensi- 
tiveness of the primary radicle 
in the bean, 155: in the com- 
mon pea, 156 ; effect of moist 
air, 180; of killing or injuring 
the primary radicle, 186, 187 ; 
circumnutation of flower-stems, 
225 ; epinasty, 268 ; movement* 
of leaflets of Trifolium incar- 
nation, 350 ; action of light in 
modifying the periodic move- 
ments of leaves, 418 ; on geotro- 
pism and heliotropism, 4^Q, n.; 
on Trop -solum majus, 458 * 



INDEX. 



589 



SARRACENIA. 

on the hypocotyls slightly helio- 
tropie, and stems strongly aphe- 
liotropic of the ivy, 453 ; he- 
liotropism of radicles, 482 ; ex- 
periments on tips of radicles 
of bean, 523, 524 ; curvature of 
the hypocotyl, 555 ; resemblance 
between plants and animals, 
571 

Sarracenia purpurea, circumnuta- 
tion of young pitcher, 227 

Saxifraga sarmentosa, circum- 
nutation of an inclined stolon, 
218 

Schrankia aculeata, nyctitropic 
movement of the pinnae, 381, 
403 

uncinata, nyctitropic move- 
ments of leaflets. 381 

Securigera coronilla, nocturnal 
movements of leaflets, 352 

Seed-capsules, burying of, 513 

Seed-coats, rupture of, 102-106 

Seedling plants, circumnutating 
movements of, 10 

Selaginella, circumnutation of, 258 

Kraussii (?), circumnutation of 

young plant, 66 

Sida napoza, depression of leaves at 
night, 322 

— — , no pulvinus, 322 

retusa, vertical rising of leaves, 

322 

rhombi folia, sleep of cotyledons, 

308 

, sleep of leaves, 314 

, vertical rising of leaves, 322 

, no pulvinus, 322 

, circumnutation and nycti- 
tropic movements of leaf of young 
plant, 322 

, nyctitropic movement of 

leaves, 397 

Siegesbeckia orientalis, sleep of 
leaves, 319, 384 

Sinapis alba, hypocotyl bending to- 
wards the light, 461 

, transmitted effect of light on 

radicles, 482, 483, 567 

~ — , growth of radioles in dark- 
ness, 486 



STAPELtA. 

Sinapis nigra, sleep of cotyledons, 
301 

Smilax aepera, tendrils aphelic- 
tropic, 451 

Smithia Pfundii, non - sensitive 
cotyledons, 127 

, hyponastic movement of the 

curved summit of the stem, 274- 
276 

, cotyledons not sleeping at 

night, 308 

, vertical movement of leaves, 

356 

sensitica, sensitiveness of coty- 
ledons to contact, 126 

, sleep of cotyledons, 308 

SopTiora chrysophylla, leaflets rise at 
ni^ht, ^68 

Solatium dulcamara, circumnuta- 
ting stems, 266 

lycopersicum, movement of 

hypocotyl, 50 

, of cotyledons, 50 

, effect of darkness, 124 

, rising of cotyledons at night 

306 
, heliotrupic movements of 

hypocotyl, 421 
, effect of an intermittent light, 

457 
, rapid heliotropism, 461 

palinacanthum, circumnu- 
tation of arched hypocotyl, 51, 
100 

, of cotyledon, 51 

, ellipses described by hypo- 
cotyl when erect, 107 

^— , nocturnal movement of coty- 
ledons, 306 

Sparganium ramosum, rhizomes of, 
189 

Sphwrophym salsola, rising of 
leaflets, 355 

Spirogyra princeps, movements of, 
259, n. 

Stahl, Dr., on the effect of JEci- 
dium on shoot, 189; on the in- 
fluence of light on swarm-spores, 
488, n. 

Stapelia sarpedon, circumnutation 
of hypocotyl, 46, 47 



590 



INDEX. 



STAPEL.IA. 


TEITICUM. 


Stapelia sarpedon, minute coty- 


Trifolium pratense, leaves exposed 


ledons, 97 


at night, 293 


Stellaria media, nocturnal move- 


repens, circumnutation of 


ment of leaves, 297 


flower-stem, 225 


Sterne, circimmutation of, 201-214 


, circumnutating and epinastic 


Stolons, or Eunners, circumnuta- 


movements of flower-stem, 276- 


tion of, 214-222, 558 


279 


Strasburger, on the effect of light 


, nyctitropic movement of 


on spores of Haematoccus, 455, n. ; 


leaves, 349 


the influence of light on the 


, circumnutation and nycti- 


swarm-spores, 488 


tropic movements of terminal 


Strawberry, stolons of the, circum- 


leaflets, 352, 353 


nutate, but nut affected by mode- 


, sleep movements, 349 


rate light, 454 


resupinatum, no pulvini to 


Strephium floribundum, circumnu- 


cotyledons, 118 


tation and nyctitropic movement 


, circumnutation of stem, 204 


of leaves, 391, 392 


, effect of exposure at night, 

295 
, cotyledons not rising at 






night, 118, 309 


T. 


, circumnutation and nycti- 


tropic movements of terminal 




leaflets, 351, 352 


Tamarindus Indica, nyctitropic 


striatum, movements of coty- 


movement of leaflets, 374 


ledons at night, 116, 118 


Transversal - heliotropi.-mus (of 


, nocturnal and diurnal move- 


Frank) or diaheliotropism, 438 


ments of cotyledons, 309-311, 


Trapa natans, unequal cotyledons, 


313 


95, n. 


, movement of the left-hand 


Tecoma radicans, stems aphelio- 


cotyledon, 316 


tropic, 451 


subterraneum, movement of 


Tephrosia caribsea, 354 


flower-heads, 71 


Terminology, 5 


, of cotyledons at night, 116, 


TJialia dealbata, sleep of leaves, 


118, 309 


389 


, circumnutation of flower-stem, 


• , lateral movement of leaves, 


224, 225 


404 


, circumnutation and nycti- 


Trichosanthes anguina, action of the 


tropic movements of leaves, 350 


peg on the radicle, 104 


, number of ellipses in 24 


, nocturnal movement of coty- 


hours, 405 


ledons, 304 


, burying its flower heads, 513, 


Trifolium, position of terminal leaf- 


514 


lets at night, 282 


, downward movement of pe- 


gldbosum, with hairs protecting 


duncle, 515 


the seed-bearing flowers, 517 


. circumnutating movement of 


qlomeratum, movement of 


peduncle, 516 


cotyledons, 309 


Trigonella Cretica, sleep of leaves^ 


'■■ incarnatum, movement of 


345 


cotyledons, 309 


Triticum repens, undergrounJ 


«— - Pannonicum, shape of first 


shoots of, become apogeotropia, 


true leaf, 350, 415 


189 



INDEX. 



591 



Tritieum vulgare, sensitiveness of 

tips of radicle to moist air, 184 
Tropoeolum majus (?), sensitiveness 

of apex of radicle to contact, 167 

, circumnutation of stem, 204 

, influence of illumination on 

nyctitropic movements, 338-340, 

344 
, heliotropic movement and 

circumnutation of epicotyl of a 

young seedling, 428, 429 
, of an old internode towards a 

lateral light, 430 
, stems of very young plants 

highly heliotropic, of old plants 

slightly apheliotropic, 453 

, effect of lateral light, 484 

minus (?), circumnutation of 

buried and arched epicotyl, 27 



U. 



Ulex, or gorse, first-formed leaf of, 
415 

Uraria lagopus, vertical sinking of 
leaflets at night, 365 

Vaucher, on the burying of the 
flower-heads of Trlfolium sub- 
terraneum, 513; on the protec- 
tion of seeds, 517 

Verbena melmdres (?), circumnuta- 
tion of stem, 210 

, apogeotropic movement of 

stem, 495 



V. 



Vicia faba, circumnutation of ra- 
dicle, 29, 30 

, of epicotyl, 31-33 

, curvature of hypocotyl, 92 

, sensitiveness of apex of ra- 
dicle, 132-134 
, of the tips of secondary ra- 
dicles, 154 

, of the primary radicle above 

the apex, 155-158 

, various experiments, 135-143 

, summary of results, 143-151 

, power of an irritant on, com- 



WILSON. 

pared with that of geotropism, 
151-154 
Vicia faba, circumnutation of leaves, 
233-235 

, circumnutaLon of terminal 

leaflet, 235 

, effect of apogeotropism, 444 

, effect of amputating the tips 

of radicles, 523 

, regeneration of tips, 526 

, short exposure to geotropio 

action, 527 

, effects of amputating the tips 

obliquely, 528 

, of cauterising the tips, 529 

, of grease on the tips, 534 

Vines, Mr., on cell growth, 3 
Vries, De, on turgescence, 2 • on 
epinasty and hyponasty, 6, 267, 
268; the protection of hypo- 
cotyls during winter, 557 ; stolons 
apheliotropic, 108 ; the nycti- 
tropio movement of leaves, 283 ; 
the position of leaves influenced 
by epinasty, their own weight and 
apogeotropism, 440 ; apogeotro- 
pism in petioles and midribs, 443 ; 
the stolons of strawberries, 454 ; 
the joints or pulvini of the Gra- 
mmes}, 502 

W. 



hygrometrica, 336-338 
Wells, ' Essay on Dew,' 284, n. 
Wiesuer, Prof., on the circumnuta- 
tion of the hypocotyl, 99, 100 ; 
on the hooked tip of climbing 
stems, 272 ; observations on the 
effect of bright sunshine un 
chlorophyll in leaves, 446; the 
effects of an intermittent light, 
457 ; on aerial roots, 486 ; on 
special adaptations, 490 
Wigandia, movement of leaves, 248 
Williamson, Prof, on leaves of 

Drosera Capensis, 414 
Wilson, Mr. A. S., on the move- 
ments of Swedish turnip leaves, 
230, 298 



592 



INDEX. 



WINKLES. 


ZUKAL. 


Winkler on the protection of seed- 


Zea Mays, geotropic movement ci 


lings, 108 


radicles, 65 


Wislaria Sinensis, leaflets depressed 


, sensitiveness of apex of ra- 


at night, 354 


dicle to contact, 177-179 


, circuranutation with lateral 


, secondary radicles, 179 


light, 452 


, heliotropic movements of 




seedling, 64, 421 


z 


, tips of radicles cauterised, 




539 


Zea Mays, circuranutation of coty- 


Zukal, on the movements of Spiru- 


lfldrai, 64 


lina, 259, n. 



THE END. 






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